Merge pull request #3728 from poettering/dynamic-users

21 files changed, 1246 insertions, 31 deletions

diff --git a/src/core/dbus-execute.c b/src/core/dbus-execute.c
index 307c3d8e7a..9c50cd93e5 100644
--- a/src/core/dbus-execute.c
+++ b/src/core/dbus-execute.c
@@ -44,6 +44,7 @@
 #endif
 #include "strv.h"
 #include "syslog-util.h"
+#include "user-util.h"
 #include "utf8.h"
 
 BUS_DEFINE_PROPERTY_GET_ENUM(bus_property_get_exec_output, exec_output, ExecOutput);
@@ -693,6 +694,7 @@ const sd_bus_vtable bus_exec_vtable[] = {
         SD_BUS_PROPERTY("AmbientCapabilities", "t", property_get_ambient_capabilities, 0, SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("User", "s", NULL, offsetof(ExecContext, user), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("Group", "s", NULL, offsetof(ExecContext, group), SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("DynamicUser", "b", bus_property_get_bool, offsetof(ExecContext, dynamic_user), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("SupplementaryGroups", "as", NULL, offsetof(ExecContext, supplementary_groups), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("PAMName", "s", NULL, offsetof(ExecContext, pam_name), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("ReadWriteDirectories", "as", NULL, offsetof(ExecContext, read_write_paths), SD_BUS_VTABLE_PROPERTY_CONST|SD_BUS_VTABLE_HIDDEN),
@@ -840,6 +842,9 @@ int bus_exec_context_set_transient_property(
                 if (r < 0)
                         return r;
 
+                if (!isempty(uu) && !valid_user_group_name_or_id(uu))
+                        return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Invalid user name: %s", uu);
+
                 if (mode != UNIT_CHECK) {
 
                         if (isempty(uu))
@@ -859,6 +864,9 @@ int bus_exec_context_set_transient_property(
                 if (r < 0)
                         return r;
 
+                if (!isempty(gg) && !valid_user_group_name_or_id(gg))
+                        return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Invalid group name: %s", gg);
+
                 if (mode != UNIT_CHECK) {
 
                         if (isempty(gg))
@@ -1061,7 +1069,8 @@ int bus_exec_context_set_transient_property(
         } else if (STR_IN_SET(name,
                               "IgnoreSIGPIPE", "TTYVHangup", "TTYReset",
                               "PrivateTmp", "PrivateDevices", "PrivateNetwork",
-                              "NoNewPrivileges", "SyslogLevelPrefix", "MemoryDenyWriteExecute", "RestrictRealtime")) {
+                              "NoNewPrivileges", "SyslogLevelPrefix", "MemoryDenyWriteExecute",
+                              "RestrictRealtime", "DynamicUser")) {
                 int b;
 
                 r = sd_bus_message_read(message, "b", &b);
@@ -1089,6 +1098,8 @@ int bus_exec_context_set_transient_property(
                                 c->memory_deny_write_execute = b;
                         else if (streq(name, "RestrictRealtime"))
                                 c->restrict_realtime = b;
+                        else if (streq(name, "DynamicUser"))
+                                c->dynamic_user = b;
 
                         unit_write_drop_in_private_format(u, mode, name, "%s=%s", name, yes_no(b));
                 }
diff --git a/src/core/dbus-manager.c b/src/core/dbus-manager.c
index d05968bd65..ef05a75a8b 100644
--- a/src/core/dbus-manager.c
+++ b/src/core/dbus-manager.c
@@ -43,6 +43,7 @@
 #include "string-util.h"
 #include "strv.h"
 #include "syslog-util.h"
+#include "user-util.h"
 #include "virt.h"
 #include "watchdog.h"
 
@@ -1511,8 +1512,8 @@ static int method_unset_and_set_environment(sd_bus_message *message, void *userd
 }
 
 static int method_set_exit_code(sd_bus_message *message, void *userdata, sd_bus_error *error) {
-        uint8_t code;
         Manager *m = userdata;
+        uint8_t code;
         int r;
 
         assert(message);
@@ -1534,6 +1535,61 @@ static int method_set_exit_code(sd_bus_message *message, void *userdata, sd_bus_
         return sd_bus_reply_method_return(message, NULL);
 }
 
+static int method_lookup_dynamic_user_by_name(sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        Manager *m = userdata;
+        const char *name;
+        uid_t uid;
+        int r;
+
+        assert(message);
+        assert(m);
+
+        r = sd_bus_message_read_basic(message, 's', &name);
+        if (r < 0)
+                return r;
+
+        if (!MANAGER_IS_SYSTEM(m))
+                return sd_bus_error_setf(error, SD_BUS_ERROR_NOT_SUPPORTED, "Dynamic users are only supported in the system instance.");
+        if (!valid_user_group_name(name))
+                return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "User name invalid: %s", name);
+
+        r = dynamic_user_lookup_name(m, name, &uid);
+        if (r == -ESRCH)
+                return sd_bus_error_setf(error, BUS_ERROR_NO_SUCH_DYNAMIC_USER, "Dynamic user %s does not exist.", name);
+        if (r < 0)
+                return r;
+
+        return sd_bus_reply_method_return(message, "u", (uint32_t) uid);
+}
+
+static int method_lookup_dynamic_user_by_uid(sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        _cleanup_free_ char *name = NULL;
+        Manager *m = userdata;
+        uid_t uid;
+        int r;
+
+        assert(message);
+        assert(m);
+
+        assert_cc(sizeof(uid) == sizeof(uint32_t));
+        r = sd_bus_message_read_basic(message, 'u', &uid);
+        if (r < 0)
+                return r;
+
+        if (!MANAGER_IS_SYSTEM(m))
+                return sd_bus_error_setf(error, SD_BUS_ERROR_NOT_SUPPORTED, "Dynamic users are only supported in the system instance.");
+        if (!uid_is_valid(uid))
+                return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "User ID invalid: " UID_FMT, uid);
+
+        r = dynamic_user_lookup_uid(m, uid, &name);
+        if (r == -ESRCH)
+                return sd_bus_error_setf(error, BUS_ERROR_NO_SUCH_DYNAMIC_USER, "Dynamic user ID " UID_FMT " does not exist.", uid);
+        if (r < 0)
+                return r;
+
+        return sd_bus_reply_method_return(message, "s", name);
+}
+
 static int list_unit_files_by_patterns(sd_bus_message *message, void *userdata, sd_bus_error *error, char **states, char **patterns) {
         _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
         Manager *m = userdata;
@@ -2199,6 +2255,8 @@ const sd_bus_vtable bus_manager_vtable[] = {
         SD_BUS_METHOD("PresetAllUnitFiles", "sbb", "a(sss)", method_preset_all_unit_files, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("AddDependencyUnitFiles", "asssbb", "a(sss)", method_add_dependency_unit_files, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("SetExitCode", "y", NULL, method_set_exit_code, SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD("LookupDynamicUserByName", "s", "u", method_lookup_dynamic_user_by_name, SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD("LookupDynamicUserByUID", "u", "s", method_lookup_dynamic_user_by_uid, SD_BUS_VTABLE_UNPRIVILEGED),
 
         SD_BUS_SIGNAL("UnitNew", "so", 0),
         SD_BUS_SIGNAL("UnitRemoved", "so", 0),
diff --git a/src/core/dynamic-user.c b/src/core/dynamic-user.c
new file mode 100644
index 0000000000..8035bee231
--- /dev/null
+++ b/src/core/dynamic-user.c
@@ -0,0 +1,763 @@
+/***
+  This file is part of systemd.
+
+  Copyright 2016 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <grp.h>
+#include <pwd.h>
+#include <sys/file.h>
+
+#include "dynamic-user.h"
+#include "fd-util.h"
+#include "fs-util.h"
+#include "parse-util.h"
+#include "random-util.h"
+#include "stdio-util.h"
+#include "string-util.h"
+#include "user-util.h"
+#include "fileio.h"
+
+/* Let's pick a UIDs within the 16bit range, so that we are compatible with containers using 16bit user namespacing. At
+ * least on Fedora normal users are allocated until UID 60000, hence do not allocate from below this. Also stay away
+ * from the upper end of the range as that is often used for overflow/nobody users. */
+#define UID_PICK_MIN ((uid_t) UINT32_C(0x0000EF00))
+#define UID_PICK_MAX ((uid_t) UINT32_C(0x0000FFEF))
+
+/* Takes a value generated randomly or by hashing and turns it into a UID in the right range */
+#define UID_CLAMP_INTO_RANGE(rnd) (((uid_t) (rnd) % (UID_PICK_MAX - UID_PICK_MIN + 1)) + UID_PICK_MIN)
+
+static DynamicUser* dynamic_user_free(DynamicUser *d) {
+        if (!d)
+                return NULL;
+
+        if (d->manager)
+                (void) hashmap_remove(d->manager->dynamic_users, d->name);
+
+        safe_close_pair(d->storage_socket);
+        free(d);
+
+        return NULL;
+}
+
+static int dynamic_user_add(Manager *m, const char *name, int storage_socket[2], DynamicUser **ret) {
+        DynamicUser *d = NULL;
+        int r;
+
+        assert(m);
+        assert(name);
+        assert(storage_socket);
+
+        r = hashmap_ensure_allocated(&m->dynamic_users, &string_hash_ops);
+        if (r < 0)
+                return r;
+
+        d = malloc0(offsetof(DynamicUser, name) + strlen(name) + 1);
+        if (!d)
+                return -ENOMEM;
+
+        strcpy(d->name, name);
+
+        d->storage_socket[0] = storage_socket[0];
+        d->storage_socket[1] = storage_socket[1];
+
+        r = hashmap_put(m->dynamic_users, d->name, d);
+        if (r < 0) {
+                free(d);
+                return r;
+        }
+
+        d->manager = m;
+
+        if (ret)
+                *ret = d;
+
+        return 0;
+}
+
+int dynamic_user_acquire(Manager *m, const char *name, DynamicUser** ret) {
+        _cleanup_close_pair_ int storage_socket[2] = { -1, -1 };
+        DynamicUser *d;
+        int r;
+
+        assert(m);
+        assert(name);
+
+        /* Return the DynamicUser structure for a specific user name. Note that this won't actually allocate a UID for
+         * it, but just prepare the data structure for it. The UID is allocated only on demand, when it's really
+         * needed, and in the child process we fork off, since allocation involves NSS checks which are not OK to do
+         * from PID 1. To allow the children and PID 1 share information about allocated UIDs we use an anonymous
+         * AF_UNIX/SOCK_DGRAM socket (called the "storage socket") that contains at most one datagram with the
+         * allocated UID number, plus an fd referencing the lock file for the UID
+         * (i.e. /run/systemd/dynamic-uid/$UID). Why involve the socket pair? So that PID 1 and all its children can
+         * share the same storage for the UID and lock fd, simply by inheriting the storage socket fds. The socket pair
+         * may exist in three different states:
+         *
+         * a) no datagram stored. This is the initial state. In this case the dynamic user was never realized.
+         *
+         * b) a datagram containing a UID stored, but no lock fd attached to it. In this case there was already a
+         *    statically assigned UID by the same name, which we are reusing.
+         *
+         * c) a datagram containing a UID stored, and a lock fd is attached to it. In this case we allocated a dynamic
+         *    UID and locked it in the file system, using the lock fd.
+         *
+         * As PID 1 and various children might access the socket pair simultaneously, and pop the datagram or push it
+         * back in any time, we also maintain a lock on the socket pair. Note one peculiarity regarding locking here:
+         * the UID lock on disk is protected via a BSD file lock (i.e. an fd-bound lock), so that the lock is kept in
+         * place as long as there's a reference to the fd open. The lock on the storage socket pair however is a POSIX
+         * file lock (i.e. a process-bound lock), as all users share the same fd of this (after all it is anonymous,
+         * nobody else could get any access to it except via our own fd) and we want to synchronize access between all
+         * processes that have access to it. */
+
+        d = hashmap_get(m->dynamic_users, name);
+        if (d) {
+                /* We already have a structure for the dynamic user, let's increase the ref count and reuse it */
+                d->n_ref++;
+                *ret = d;
+                return 0;
+        }
+
+        if (!valid_user_group_name_or_id(name))
+                return -EINVAL;
+
+        if (socketpair(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0, storage_socket) < 0)
+                return -errno;
+
+        r = dynamic_user_add(m, name, storage_socket, &d);
+        if (r < 0)
+                return r;
+
+        storage_socket[0] = storage_socket[1] = -1;
+
+        if (ret) {
+                d->n_ref++;
+                *ret = d;
+        }
+
+        return 1;
+}
+
+static int pick_uid(const char *name, uid_t *ret_uid) {
+
+        static const uint8_t hash_key[] = {
+                0x37, 0x53, 0x7e, 0x31, 0xcf, 0xce, 0x48, 0xf5,
+                0x8a, 0xbb, 0x39, 0x57, 0x8d, 0xd9, 0xec, 0x59
+        };
+
+        unsigned n_tries = 100;
+        uid_t candidate;
+        int r;
+
+        /* A static user by this name does not exist yet. Let's find a free ID then, and use that. We start with a UID
+         * generated as hash from the user name. */
+        candidate = UID_CLAMP_INTO_RANGE(siphash24(name, strlen(name), hash_key));
+
+        (void) mkdir("/run/systemd/dynamic-uid", 0755);
+
+        for (;;) {
+                char lock_path[strlen("/run/systemd/dynamic-uid/") + DECIMAL_STR_MAX(uid_t) + 1];
+                _cleanup_close_ int lock_fd = -1;
+                ssize_t l;
+
+                if (--n_tries <= 0) /* Give up retrying eventually */
+                        return -EBUSY;
+
+                if (candidate < UID_PICK_MIN || candidate > UID_PICK_MAX)
+                        goto next;
+
+                xsprintf(lock_path, "/run/systemd/dynamic-uid/" UID_FMT, candidate);
+
+                for (;;) {
+                        struct stat st;
+
+                        lock_fd = open(lock_path, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
+                        if (lock_fd < 0)
+                                return -errno;
+
+                        r = flock(lock_fd, LOCK_EX|LOCK_NB); /* Try to get a BSD file lock on the UID lock file */
+                        if (r < 0) {
+                                if (errno == EBUSY || errno == EAGAIN)
+                                        goto next; /* already in use */
+
+                                return -errno;
+                        }
+
+                        if (fstat(lock_fd, &st) < 0)
+                                return -errno;
+                        if (st.st_nlink > 0)
+                                break;
+
+                        /* Oh, bummer, we got got the lock, but the file was unlinked between the time we opened it and
+                         * got the lock. Close it, and try again. */
+                        lock_fd = safe_close(lock_fd);
+                }
+
+                /* Some superficial check whether this UID/GID might already be taken by some static user */
+                if (getpwuid(candidate) || getgrgid((gid_t) candidate)) {
+                        (void) unlink(lock_path);
+                        goto next;
+                }
+
+                /* Let's store the user name in the lock file, so that we can use it for looking up the username for a UID */
+                l = pwritev(lock_fd,
+                            (struct iovec[2]) {
+                                    { .iov_base = (char*) name, .iov_len = strlen(name) },
+                                    { .iov_base = (char[1]) { '\n' }, .iov_len = 1 }
+                            }, 2, 0);
+                if (l < 0) {
+                        (void) unlink(lock_path);
+                        return -errno;
+                }
+
+                (void) ftruncate(lock_fd, l);
+
+                *ret_uid = candidate;
+                r = lock_fd;
+                lock_fd = -1;
+
+                return r;
+
+        next:
+                /* Pick another random UID, and see if that works for us. */
+                random_bytes(&candidate, sizeof(candidate));
+                candidate = UID_CLAMP_INTO_RANGE(candidate);
+        }
+}
+
+static int dynamic_user_pop(DynamicUser *d, uid_t *ret_uid, int *ret_lock_fd) {
+        uid_t uid = UID_INVALID;
+        struct iovec iov = {
+                .iov_base = &uid,
+                .iov_len = sizeof(uid),
+        };
+        union {
+                struct cmsghdr cmsghdr;
+                uint8_t buf[CMSG_SPACE(sizeof(int))];
+        } control = {};
+        struct msghdr mh = {
+                .msg_control = &control,
+                .msg_controllen = sizeof(control),
+                .msg_iov = &iov,
+                .msg_iovlen = 1,
+        };
+        struct cmsghdr *cmsg;
+
+        ssize_t k;
+        int lock_fd = -1;
+
+        assert(d);
+        assert(ret_uid);
+        assert(ret_lock_fd);
+
+        /* Read the UID and lock fd that is stored in the storage AF_UNIX socket. This should be called with the lock
+         * on the socket taken. */
+
+        k = recvmsg(d->storage_socket[0], &mh, MSG_DONTWAIT|MSG_NOSIGNAL|MSG_CMSG_CLOEXEC);
+        if (k < 0)
+                return -errno;
+
+        cmsg = cmsg_find(&mh, SOL_SOCKET, SCM_RIGHTS, CMSG_LEN(sizeof(int)));
+        if (cmsg)
+                lock_fd = *(int*) CMSG_DATA(cmsg);
+        else
+                cmsg_close_all(&mh); /* just in case... */
+
+        *ret_uid = uid;
+        *ret_lock_fd = lock_fd;
+
+        return 0;
+}
+
+static int dynamic_user_push(DynamicUser *d, uid_t uid, int lock_fd) {
+        struct iovec iov = {
+                .iov_base = &uid,
+                .iov_len = sizeof(uid),
+        };
+        union {
+                struct cmsghdr cmsghdr;
+                uint8_t buf[CMSG_SPACE(sizeof(int))];
+        } control = {};
+        struct msghdr mh = {
+                .msg_control = &control,
+                .msg_controllen = sizeof(control),
+                .msg_iov = &iov,
+                .msg_iovlen = 1,
+        };
+        ssize_t k;
+
+        assert(d);
+
+        /* Store the UID and lock_fd in the storage socket. This should be called with the socket pair lock taken. */
+
+        if (lock_fd >= 0) {
+                struct cmsghdr *cmsg;
+
+                cmsg = CMSG_FIRSTHDR(&mh);
+                cmsg->cmsg_level = SOL_SOCKET;
+                cmsg->cmsg_type = SCM_RIGHTS;
+                cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+                memcpy(CMSG_DATA(cmsg), &lock_fd, sizeof(int));
+
+                mh.msg_controllen = CMSG_SPACE(sizeof(int));
+        } else {
+                mh.msg_control = NULL;
+                mh.msg_controllen = 0;
+        }
+
+        k = sendmsg(d->storage_socket[1], &mh, MSG_DONTWAIT|MSG_NOSIGNAL);
+        if (k < 0)
+                return -errno;
+
+        return 0;
+}
+
+static void unlink_uid_lock(int lock_fd, uid_t uid) {
+        char lock_path[strlen("/run/systemd/dynamic-uid/") + DECIMAL_STR_MAX(uid_t) + 1];
+
+        if (lock_fd < 0)
+                return;
+
+        xsprintf(lock_path, "/run/systemd/dynamic-uid/" UID_FMT, uid);
+        (void) unlink_noerrno(lock_path);
+}
+
+int dynamic_user_realize(DynamicUser *d, uid_t *ret) {
+
+        _cleanup_close_ int etc_passwd_lock_fd = -1, uid_lock_fd = -1;
+        uid_t uid = UID_INVALID;
+        int r;
+
+        assert(d);
+
+        /* Acquire a UID for the user name. This will allocate a UID for the user name if the user doesn't exist
+         * yet. If it already exists its existing UID/GID will be reused. */
+
+        if (lockf(d->storage_socket[0], F_LOCK, 0) < 0)
+                return -errno;
+
+        r = dynamic_user_pop(d, &uid, &uid_lock_fd);
+        if (r < 0) {
+                int new_uid_lock_fd;
+                uid_t new_uid;
+
+                if (r != -EAGAIN)
+                        goto finish;
+
+                /* OK, nothing stored yet, let's try to find something useful. While we are working on this release the
+                 * lock however, so that nobody else blocks on our NSS lookups. */
+                (void) lockf(d->storage_socket[0], F_ULOCK, 0);
+
+                /* Let's see if a proper, static user or group by this name exists. Try to take the lock on
+                 * /etc/passwd, if that fails with EROFS then /etc is read-only. In that case it's fine if we don't
+                 * take the lock, given that users can't be added there anyway in this case. */
+                etc_passwd_lock_fd = take_etc_passwd_lock(NULL);
+                if (etc_passwd_lock_fd < 0 && etc_passwd_lock_fd != -EROFS)
+                        return etc_passwd_lock_fd;
+
+                /* First, let's parse this as numeric UID */
+                r = parse_uid(d->name, &uid);
+                if (r < 0) {
+                        struct passwd *p;
+                        struct group *g;
+
+                        /* OK, this is not a numeric UID. Let's see if there's a user by this name */
+                        p = getpwnam(d->name);
+                        if (p)
+                                uid = p->pw_uid;
+
+                        /* Let's see if there's a group by this name */
+                        g = getgrnam(d->name);
+                        if (g) {
+                                /* If the UID/GID of the user/group of the same don't match, refuse operation */
+                                if (uid != UID_INVALID && uid != (uid_t) g->gr_gid)
+                                        return -EILSEQ;
+
+                                uid = (uid_t) g->gr_gid;
+                        }
+                }
+
+                if (uid == UID_INVALID) {
+                        /* No static UID assigned yet, excellent. Let's pick a new dynamic one, and lock it. */
+
+                        uid_lock_fd = pick_uid(d->name, &uid);
+                        if (uid_lock_fd < 0)
+                                return uid_lock_fd;
+                }
+
+                /* So, we found a working UID/lock combination. Let's see if we actually still need it. */
+                if (lockf(d->storage_socket[0], F_LOCK, 0) < 0) {
+                        unlink_uid_lock(uid_lock_fd, uid);
+                        return -errno;
+                }
+
+                r = dynamic_user_pop(d, &new_uid, &new_uid_lock_fd);
+                if (r < 0) {
+                        if (r != -EAGAIN) {
+                                /* OK, something bad happened, let's get rid of the bits we acquired. */
+                                unlink_uid_lock(uid_lock_fd, uid);
+                                goto finish;
+                        }
+
+                        /* Great! Nothing is stored here, still. Store our newly acquired data. */
+                } else {
+                        /* Hmm, so as it appears there's now something stored in the storage socket. Throw away what we
+                         * acquired, and use what's stored now. */
+
+                        unlink_uid_lock(uid_lock_fd, uid);
+                        safe_close(uid_lock_fd);
+
+                        uid = new_uid;
+                        uid_lock_fd = new_uid_lock_fd;
+                }
+        }
+
+        /* If the UID/GID was already allocated dynamically, push the data we popped out back in. If it was already
+         * allocated statically, push the UID back too, but do not push the lock fd in. If we allocated the UID
+         * dynamically right here, push that in along with the lock fd for it. */
+        r = dynamic_user_push(d, uid, uid_lock_fd);
+        if (r < 0)
+                goto finish;
+
+        *ret = uid;
+        r = 0;
+
+finish:
+        (void) lockf(d->storage_socket[0], F_ULOCK, 0);
+        return r;
+}
+
+int dynamic_user_current(DynamicUser *d, uid_t *ret) {
+        _cleanup_close_ int lock_fd = -1;
+        uid_t uid;
+        int r;
+
+        assert(d);
+        assert(ret);
+
+        /* Get the currently assigned UID for the user, if there's any. This simply pops the data from the storage socket, and pushes it back in right-away. */
+
+        if (lockf(d->storage_socket[0], F_LOCK, 0) < 0)
+                return -errno;
+
+        r = dynamic_user_pop(d, &uid, &lock_fd);
+        if (r < 0)
+                goto finish;
+
+        r = dynamic_user_push(d, uid, lock_fd);
+        if (r < 0)
+                goto finish;
+
+        *ret = uid;
+        r = 0;
+
+finish:
+        (void) lockf(d->storage_socket[0], F_ULOCK, 0);
+        return r;
+}
+
+DynamicUser* dynamic_user_ref(DynamicUser *d) {
+        if (!d)
+                return NULL;
+
+        assert(d->n_ref > 0);
+        d->n_ref++;
+
+        return d;
+}
+
+DynamicUser* dynamic_user_unref(DynamicUser *d) {
+        if (!d)
+                return NULL;
+
+        /* Note that this doesn't actually release any resources itself. If a dynamic user should be fully destroyed
+         * and its UID released, use dynamic_user_destroy() instead. NB: the dynamic user table may contain entries
+         * with no references, which is commonly the case right before a daemon reload. */
+
+        assert(d->n_ref > 0);
+        d->n_ref--;
+
+        return NULL;
+}
+
+static int dynamic_user_close(DynamicUser *d) {
+        _cleanup_close_ int lock_fd = -1;
+        uid_t uid;
+        int r;
+
+        /* Release the user ID, by releasing the lock on it, and emptying the storage socket. After this the user is
+         * unrealized again, much like it was after it the DynamicUser object was first allocated. */
+
+        if (lockf(d->storage_socket[0], F_LOCK, 0) < 0)
+                return -errno;
+
+        r = dynamic_user_pop(d, &uid, &lock_fd);
+        if (r == -EAGAIN) {
+                /* User wasn't realized yet, nothing to do. */
+                r = 0;
+                goto finish;
+        }
+        if (r < 0)
+                goto finish;
+
+        /* This dynamic user was realized and dynamically allocated. In this case, let's remove the lock file. */
+        unlink_uid_lock(lock_fd, uid);
+        r = 1;
+
+finish:
+        (void) lockf(d->storage_socket[0], F_ULOCK, 0);
+        return r;
+}
+
+DynamicUser* dynamic_user_destroy(DynamicUser *d) {
+        if (!d)
+                return NULL;
+
+        /* Drop a reference to a DynamicUser object, and destroy the user completely if this was the last
+         * reference. This is called whenever a service is shut down and wants its dynamic UID gone. Note that
+         * dynamic_user_unref() is what is called whenever a service is simply freed, for example during a reload
+         * cycle, where the dynamic users should not be destroyed, but our datastructures should. */
+
+        dynamic_user_unref(d);
+
+        if (d->n_ref > 0)
+                return NULL;
+
+        (void) dynamic_user_close(d);
+        return dynamic_user_free(d);
+}
+
+int dynamic_user_serialize(Manager *m, FILE *f, FDSet *fds) {
+        DynamicUser *d;
+        Iterator i;
+
+        assert(m);
+        assert(f);
+        assert(fds);
+
+        /* Dump the dynamic user database into the manager serialization, to deal with daemon reloads. */
+
+        HASHMAP_FOREACH(d, m->dynamic_users, i) {
+                int copy0, copy1;
+
+                copy0 = fdset_put_dup(fds, d->storage_socket[0]);
+                if (copy0 < 0)
+                        return copy0;
+
+                copy1 = fdset_put_dup(fds, d->storage_socket[1]);
+                if (copy1 < 0)
+                        return copy1;
+
+                fprintf(f, "dynamic-user=%s %i %i\n", d->name, copy0, copy1);
+        }
+
+        return 0;
+}
+
+void dynamic_user_deserialize_one(Manager *m, const char *value, FDSet *fds) {
+        _cleanup_free_ char *name = NULL, *s0 = NULL, *s1 = NULL;
+        int r, fd0, fd1;
+
+        assert(m);
+        assert(value);
+        assert(fds);
+
+        /* Parse the serialization again, after a daemon reload */
+
+        r = extract_many_words(&value, NULL, 0, &name, &s0, &s1, NULL);
+        if (r != 3 || !isempty(value)) {
+                log_debug("Unable to parse dynamic user line.");
+                return;
+        }
+
+        if (safe_atoi(s0, &fd0) < 0 || !fdset_contains(fds, fd0)) {
+                log_debug("Unable to process dynamic user fd specification.");
+                return;
+        }
+
+        if (safe_atoi(s1, &fd1) < 0 || !fdset_contains(fds, fd1)) {
+                log_debug("Unable to process dynamic user fd specification.");
+                return;
+        }
+
+        r = dynamic_user_add(m, name, (int[]) { fd0, fd1 }, NULL);
+        if (r < 0) {
+                log_debug_errno(r, "Failed to add dynamic user: %m");
+                return;
+        }
+
+        (void) fdset_remove(fds, fd0);
+        (void) fdset_remove(fds, fd1);
+}
+
+void dynamic_user_vacuum(Manager *m, bool close_user) {
+        DynamicUser *d;
+        Iterator i;
+
+        assert(m);
+
+        /* Empty the dynamic user database, optionally cleaning up orphaned dynamic users, i.e. destroy and free users
+         * to which no reference exist. This is called after a daemon reload finished, in order to destroy users which
+         * might not be referenced anymore. */
+
+        HASHMAP_FOREACH(d, m->dynamic_users, i) {
+                if (d->n_ref > 0)
+                        continue;
+
+                if (close_user) {
+                        log_debug("Removing orphaned dynamic user %s", d->name);
+                        (void) dynamic_user_close(d);
+                }
+
+                dynamic_user_free(d);
+        }
+}
+
+int dynamic_user_lookup_uid(Manager *m, uid_t uid, char **ret) {
+        char lock_path[strlen("/run/systemd/dynamic-uid/") + DECIMAL_STR_MAX(uid_t) + 1];
+        _cleanup_free_ char *user = NULL;
+        uid_t check_uid;
+        int r;
+
+        assert(m);
+        assert(ret);
+
+        /* A friendly way to translate a dynamic user's UID into a his name. */
+
+        if (uid < UID_PICK_MIN)
+                return -ESRCH;
+        if (uid > UID_PICK_MAX)
+                return -ESRCH;
+
+        xsprintf(lock_path, "/run/systemd/dynamic-uid/" UID_FMT, uid);
+        r = read_one_line_file(lock_path, &user);
+        if (r == -ENOENT)
+                return -ESRCH;
+        if (r < 0)
+                return r;
+
+        /* The lock file might be stale, hence let's verify the data before we return it */
+        r = dynamic_user_lookup_name(m, user, &check_uid);
+        if (r < 0)
+                return r;
+        if (check_uid != uid) /* lock file doesn't match our own idea */
+                return -ESRCH;
+
+        *ret = user;
+        user = NULL;
+
+        return 0;
+}
+
+int dynamic_user_lookup_name(Manager *m, const char *name, uid_t *ret) {
+        DynamicUser *d;
+        int r;
+
+        assert(m);
+        assert(name);
+        assert(ret);
+
+        /* A friendly call for translating a dynamic user's name into its UID */
+
+        d = hashmap_get(m->dynamic_users, name);
+        if (!d)
+                return -ESRCH;
+
+        r = dynamic_user_current(d, ret);
+        if (r == -EAGAIN) /* not realized yet? */
+                return -ESRCH;
+
+        return r;
+}
+
+int dynamic_creds_acquire(DynamicCreds *creds, Manager *m, const char *user, const char *group) {
+        bool acquired = false;
+        int r;
+
+        assert(creds);
+        assert(m);
+
+        /* A DynamicUser object encapsulates an allocation of both a UID and a GID for a specific name. However, some
+         * services use different user and groups. For cases like that there's DynamicCreds containing a pair of user
+         * and group. This call allocates a pair. */
+
+        if (!creds->user && user) {
+                r = dynamic_user_acquire(m, user, &creds->user);
+                if (r < 0)
+                        return r;
+
+                acquired = true;
+        }
+
+        if (!creds->group) {
+
+                if (creds->user && (!group || streq_ptr(user, group)))
+                        creds->group = dynamic_user_ref(creds->user);
+                else {
+                        r = dynamic_user_acquire(m, group, &creds->group);
+                        if (r < 0) {
+                                if (acquired)
+                                        creds->user = dynamic_user_unref(creds->user);
+                                return r;
+                        }
+                }
+        }
+
+        return 0;
+}
+
+int dynamic_creds_realize(DynamicCreds *creds, uid_t *uid, gid_t *gid) {
+        uid_t u = UID_INVALID;
+        gid_t g = GID_INVALID;
+        int r;
+
+        assert(creds);
+        assert(uid);
+        assert(gid);
+
+        /* Realize both the referenced user and group */
+
+        if (creds->user) {
+                r = dynamic_user_realize(creds->user, &u);
+                if (r < 0)
+                        return r;
+        }
+
+        if (creds->group && creds->group != creds->user) {
+                r = dynamic_user_realize(creds->group, &g);
+                if (r < 0)
+                        return r;
+        } else
+                g = u;
+
+        *uid = u;
+        *gid = g;
+
+        return 0;
+}
+
+void dynamic_creds_unref(DynamicCreds *creds) {
+        assert(creds);
+
+        creds->user = dynamic_user_unref(creds->user);
+        creds->group = dynamic_user_unref(creds->group);
+}
+
+void dynamic_creds_destroy(DynamicCreds *creds) {
+        assert(creds);
+
+        creds->user = dynamic_user_destroy(creds->user);
+        creds->group = dynamic_user_destroy(creds->group);
+}
diff --git a/src/core/dynamic-user.h b/src/core/dynamic-user.h
new file mode 100644
index 0000000000..0b8bce1a72
--- /dev/null
+++ b/src/core/dynamic-user.h
@@ -0,0 +1,66 @@
+#pragma once
+
+/***
+  This file is part of systemd.
+
+  Copyright 2016 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+typedef struct DynamicUser DynamicUser;
+
+typedef struct DynamicCreds {
+        /* A combination of a dynamic user and group */
+        DynamicUser *user;
+        DynamicUser *group;
+} DynamicCreds;
+
+#include "manager.h"
+
+/* Note that this object always allocates a pair of user and group under the same name, even if one of them isn't
+ * used. This means, if you want to allocate a group and user pair, and they might have two different names, then you
+ * need to allocated two of these objects. DynamicCreds below makes that easy. */
+struct DynamicUser {
+        int n_ref;
+        Manager *manager;
+
+        /* An AF_UNIX socket pair that contains a datagram containing both the numeric ID assigned, as well as a lock
+         * file fd locking the user ID we picked. */
+        int storage_socket[2];
+
+        char name[];
+};
+
+int dynamic_user_acquire(Manager *m, const char *name, DynamicUser **ret);
+
+int dynamic_user_realize(DynamicUser *d, uid_t *ret);
+int dynamic_user_current(DynamicUser *d, uid_t *ret);
+
+DynamicUser* dynamic_user_ref(DynamicUser *d);
+DynamicUser* dynamic_user_unref(DynamicUser *d);
+DynamicUser* dynamic_user_destroy(DynamicUser *d);
+
+int dynamic_user_serialize(Manager *m, FILE *f, FDSet *fds);
+void dynamic_user_deserialize_one(Manager *m, const char *value, FDSet *fds);
+void dynamic_user_vacuum(Manager *m, bool close_user);
+
+int dynamic_user_lookup_uid(Manager *m, uid_t uid, char **ret);
+int dynamic_user_lookup_name(Manager *m, const char *name, uid_t *ret);
+
+int dynamic_creds_acquire(DynamicCreds *creds, Manager *m, const char *user, const char *group);
+int dynamic_creds_realize(DynamicCreds *creds, uid_t *uid, gid_t *gid);
+
+void dynamic_creds_unref(DynamicCreds *creds);
+void dynamic_creds_destroy(DynamicCreds *creds);
diff --git a/src/core/execute.c b/src/core/execute.c
index 7c178b97c3..26e9cd5339 100644
--- a/src/core/execute.c
+++ b/src/core/execute.c
@@ -1526,14 +1526,28 @@ static bool exec_needs_mount_namespace(
         return false;
 }
 
+static void append_socket_pair(int *array, unsigned *n, int pair[2]) {
+        assert(array);
+        assert(n);
+
+        if (!pair)
+                return;
+
+        if (pair[0] >= 0)
+                array[(*n)++] = pair[0];
+        if (pair[1] >= 0)
+                array[(*n)++] = pair[1];
+}
+
 static int close_remaining_fds(
                 const ExecParameters *params,
                 ExecRuntime *runtime,
+                DynamicCreds *dcreds,
                 int socket_fd,
                 int *fds, unsigned n_fds) {
 
         unsigned n_dont_close = 0;
-        int dont_close[n_fds + 7];
+        int dont_close[n_fds + 11];
 
         assert(params);
 
@@ -1551,11 +1565,14 @@ static int close_remaining_fds(
                 n_dont_close += n_fds;
         }
 
-        if (runtime) {
-                if (runtime->netns_storage_socket[0] >= 0)
-                        dont_close[n_dont_close++] = runtime->netns_storage_socket[0];
-                if (runtime->netns_storage_socket[1] >= 0)
-                        dont_close[n_dont_close++] = runtime->netns_storage_socket[1];
+        if (runtime)
+                append_socket_pair(dont_close, &n_dont_close, runtime->netns_storage_socket);
+
+        if (dcreds) {
+                if (dcreds->user)
+                        append_socket_pair(dont_close, &n_dont_close, dcreds->user->storage_socket);
+                if (dcreds->group)
+                        append_socket_pair(dont_close, &n_dont_close, dcreds->group->storage_socket);
         }
 
         return close_all_fds(dont_close, n_dont_close);
@@ -1567,6 +1584,7 @@ static int exec_child(
                 const ExecContext *context,
                 const ExecParameters *params,
                 ExecRuntime *runtime,
+                DynamicCreds *dcreds,
                 char **argv,
                 int socket_fd,
                 int *fds, unsigned n_fds,
@@ -1617,7 +1635,7 @@ static int exec_child(
 
         log_forget_fds();
 
-        r = close_remaining_fds(params, runtime, socket_fd, fds, n_fds);
+        r = close_remaining_fds(params, runtime, dcreds, socket_fd, fds, n_fds);
         if (r < 0) {
                 *exit_status = EXIT_FDS;
                 return r;
@@ -1650,25 +1668,48 @@ static int exec_child(
                 }
         }
 
-        if (context->user) {
-                username = context->user;
-                r = get_user_creds(&username, &uid, &gid, &home, &shell);
+        if (context->dynamic_user && dcreds) {
+
+                /* Make sure we bypass our own NSS module for any NSS checks */
+                if (putenv((char*) "SYSTEMD_NSS_DYNAMIC_BYPASS=1") != 0) {
+                        *exit_status = EXIT_USER;
+                        return -errno;
+                }
+
+                r = dynamic_creds_realize(dcreds, &uid, &gid);
                 if (r < 0) {
                         *exit_status = EXIT_USER;
                         return r;
                 }
-        }
 
-        if (context->group) {
-                const char *g = context->group;
+                if (uid == UID_INVALID || gid == GID_INVALID) {
+                        *exit_status = EXIT_USER;
+                        return -ESRCH;
+                }
 
-                r = get_group_creds(&g, &gid);
-                if (r < 0) {
-                        *exit_status = EXIT_GROUP;
-                        return r;
+                if (dcreds->user)
+                        username = dcreds->user->name;
+
+        } else {
+                if (context->user) {
+                        username = context->user;
+                        r = get_user_creds(&username, &uid, &gid, &home, &shell);
+                        if (r < 0) {
+                                *exit_status = EXIT_USER;
+                                return r;
+                        }
                 }
-        }
 
+                if (context->group) {
+                        const char *g = context->group;
+
+                        r = get_group_creds(&g, &gid);
+                        if (r < 0) {
+                                *exit_status = EXIT_GROUP;
+                                return r;
+                        }
+                }
+        }
 
         /* If a socket is connected to STDIN/STDOUT/STDERR, we
          * must sure to drop O_NONBLOCK */
@@ -2192,6 +2233,7 @@ int exec_spawn(Unit *unit,
                const ExecContext *context,
                const ExecParameters *params,
                ExecRuntime *runtime,
+               DynamicCreds *dcreds,
                pid_t *ret) {
 
         _cleanup_strv_free_ char **files_env = NULL;
@@ -2250,6 +2292,7 @@ int exec_spawn(Unit *unit,
                                context,
                                params,
                                runtime,
+                               dcreds,
                                argv,
                                socket_fd,
                                fds, n_fds,
@@ -2723,6 +2766,8 @@ void exec_context_dump(ExecContext *c, FILE* f, const char *prefix) {
         if (c->group)
                 fprintf(f, "%sGroup: %s\n", prefix, c->group);
 
+        fprintf(f, "%sDynamicUser: %s\n", prefix, yes_no(c->dynamic_user));
+
         if (strv_length(c->supplementary_groups) > 0) {
                 fprintf(f, "%sSupplementaryGroups:", prefix);
                 strv_fprintf(f, c->supplementary_groups);
diff --git a/src/core/execute.h b/src/core/execute.h
index 189c4d0999..48cc18fbb3 100644
--- a/src/core/execute.h
+++ b/src/core/execute.h
@@ -92,6 +92,8 @@ struct ExecRuntime {
         char *tmp_dir;
         char *var_tmp_dir;
 
+        /* An AF_UNIX socket pair, that contains a datagram containing a file descriptor referring to the network
+         * namespace. */
         int netns_storage_socket[2];
 };
 
@@ -174,6 +176,8 @@ struct ExecContext {
 
         bool no_new_privileges;
 
+        bool dynamic_user;
+
         /* This is not exposed to the user but available
          * internally. We need it to make sure that whenever we spawn
          * /usr/bin/mount it is run in the same process group as us so
@@ -235,12 +239,14 @@ struct ExecParameters {
 };
 
 #include "unit.h"
+#include "dynamic-user.h"
 
 int exec_spawn(Unit *unit,
                ExecCommand *command,
                const ExecContext *context,
                const ExecParameters *exec_params,
                ExecRuntime *runtime,
+               DynamicCreds *dynamic_creds,
                pid_t *ret);
 
 void exec_command_done(ExecCommand *c);
diff --git a/src/core/load-fragment-gperf.gperf.m4 b/src/core/load-fragment-gperf.gperf.m4
index 6a5c16a000..c9cdbe8ba7 100644
--- a/src/core/load-fragment-gperf.gperf.m4
+++ b/src/core/load-fragment-gperf.gperf.m4
@@ -19,9 +19,9 @@ m4_dnl Define the context options only once
 m4_define(`EXEC_CONTEXT_CONFIG_ITEMS',
 `$1.WorkingDirectory,            config_parse_working_directory,     0,                             offsetof($1, exec_context)
 $1.RootDirectory,                config_parse_unit_path_printf,      0,                             offsetof($1, exec_context.root_directory)
-$1.User,                         config_parse_unit_string_printf,    0,                             offsetof($1, exec_context.user)
-$1.Group,                        config_parse_unit_string_printf,    0,                             offsetof($1, exec_context.group)
-$1.SupplementaryGroups,          config_parse_strv,                  0,                             offsetof($1, exec_context.supplementary_groups)
+$1.User,                         config_parse_user_group,            0,                             offsetof($1, exec_context.user)
+$1.Group,                        config_parse_user_group,            0,                             offsetof($1, exec_context.group)
+$1.SupplementaryGroups,          config_parse_user_group_strv,       0,                             offsetof($1, exec_context.supplementary_groups)
 $1.Nice,                         config_parse_exec_nice,             0,                             offsetof($1, exec_context)
 $1.OOMScoreAdjust,               config_parse_exec_oom_score_adjust, 0,                             offsetof($1, exec_context)
 $1.IOSchedulingClass,            config_parse_exec_io_class,         0,                             offsetof($1, exec_context)
@@ -34,6 +34,7 @@ $1.UMask,                        config_parse_mode,                  0,
 $1.Environment,                  config_parse_environ,               0,                             offsetof($1, exec_context.environment)
 $1.EnvironmentFile,              config_parse_unit_env_file,         0,                             offsetof($1, exec_context.environment_files)
 $1.PassEnvironment,              config_parse_pass_environ,          0,                             offsetof($1, exec_context.pass_environment)
+$1.DynamicUser,                  config_parse_bool,                  0,                             offsetof($1, exec_context.dynamic_user)
 $1.StandardInput,                config_parse_input,                 0,                             offsetof($1, exec_context.std_input)
 $1.StandardOutput,               config_parse_output,                0,                             offsetof($1, exec_context.std_output)
 $1.StandardError,                config_parse_output,                0,                             offsetof($1, exec_context.std_error)
@@ -285,8 +286,8 @@ Socket.ExecStartPost,            config_parse_exec,                  SOCKET_EXEC
 Socket.ExecStopPre,              config_parse_exec,                  SOCKET_EXEC_STOP_PRE,          offsetof(Socket, exec_command)
 Socket.ExecStopPost,             config_parse_exec,                  SOCKET_EXEC_STOP_POST,         offsetof(Socket, exec_command)
 Socket.TimeoutSec,               config_parse_sec,                   0,                             offsetof(Socket, timeout_usec)
-Socket.SocketUser,               config_parse_unit_string_printf,    0,                             offsetof(Socket, user)
-Socket.SocketGroup,              config_parse_unit_string_printf,    0,                             offsetof(Socket, group)
+Socket.SocketUser,               config_parse_user_group,            0,                             offsetof(Socket, user)
+Socket.SocketGroup,              config_parse_user_group,            0,                             offsetof(Socket, group)
 Socket.SocketMode,               config_parse_mode,                  0,                             offsetof(Socket, socket_mode)
 Socket.DirectoryMode,            config_parse_mode,                  0,                             offsetof(Socket, directory_mode)
 Socket.Accept,                   config_parse_bool,                  0,                             offsetof(Socket, accept)
diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index a36953f766..e8cb3a4249 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -64,6 +64,7 @@
 #include "unit-name.h"
 #include "unit-printf.h"
 #include "unit.h"
+#include "user-util.h"
 #include "utf8.h"
 #include "web-util.h"
 
@@ -1763,6 +1764,123 @@ int config_parse_sec_fix_0(
         return 0;
 }
 
+int config_parse_user_group(
+                const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        char **user = data, *n;
+        Unit *u = userdata;
+        int r;
+
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+        assert(u);
+
+        if (isempty(rvalue))
+                n = NULL;
+        else {
+                _cleanup_free_ char *k = NULL;
+
+                r = unit_full_printf(u, rvalue, &k);
+                if (r < 0) {
+                        log_syntax(unit, LOG_ERR, filename, line, r, "Failed to resolve unit specifiers in %s, ignoring: %m", rvalue);
+                        return 0;
+                }
+
+                if (!valid_user_group_name_or_id(k)) {
+                        log_syntax(unit, LOG_ERR, filename, line, 0, "Invalid user/group name or numeric ID, ignoring: %s", k);
+                        return 0;
+                }
+
+                n = k;
+                k = NULL;
+        }
+
+        free(*user);
+        *user = n;
+
+        return 0;
+}
+
+int config_parse_user_group_strv(
+                const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        char ***users = data;
+        Unit *u = userdata;
+        const char *p;
+        int r;
+
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+        assert(u);
+
+        if (isempty(rvalue)) {
+                char **empty;
+
+                empty = new0(char*, 1);
+                if (!empty)
+                        return log_oom();
+
+                strv_free(*users);
+                *users = empty;
+
+                return 0;
+        }
+
+        p = rvalue;
+        for (;;) {
+                _cleanup_free_ char *word = NULL, *k = NULL;
+
+                r = extract_first_word(&p, &word, WHITESPACE, 0);
+                if (r == 0)
+                        break;
+                if (r == -ENOMEM)
+                        return log_oom();
+                if (r < 0) {
+                        log_syntax(unit, LOG_ERR, filename, line, r, "Invalid syntax, ignoring: %s", rvalue);
+                        break;
+                }
+
+                r = unit_full_printf(u, word, &k);
+                if (r < 0) {
+                        log_syntax(unit, LOG_ERR, filename, line, r, "Failed to resolve unit specifiers in %s, ignoring: %m", word);
+                        continue;
+                }
+
+                if (!valid_user_group_name_or_id(k)) {
+                        log_syntax(unit, LOG_ERR, filename, line, 0, "Invalid user/group name or numeric ID, ignoring: %s", k);
+                        continue;
+                }
+
+                r = strv_push(users, k);
+                if (r < 0)
+                        return log_oom();
+
+                k = NULL;
+        }
+
+        return 0;
+}
+
 int config_parse_busname_service(
                 const char *unit,
                 const char *filename,
diff --git a/src/core/load-fragment.h b/src/core/load-fragment.h
index b36a2e3a02..213bce55a7 100644
--- a/src/core/load-fragment.h
+++ b/src/core/load-fragment.h
@@ -111,6 +111,8 @@ int config_parse_exec_utmp_mode(const char *unit, const char *filename, unsigned
 int config_parse_working_directory(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 int config_parse_fdname(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 int config_parse_sec_fix_0(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
+int config_parse_user_group(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
+int config_parse_user_group_strv(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 
 /* gperf prototypes */
 const struct ConfigPerfItem* load_fragment_gperf_lookup(const char *key, unsigned length);
diff --git a/src/core/manager.c b/src/core/manager.c
index 4d84a0b37e..e41b65da50 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -1004,6 +1004,9 @@ Manager* manager_free(Manager *m) {
 
         bus_done(m);
 
+        dynamic_user_vacuum(m, false);
+        hashmap_free(m->dynamic_users);
+
         hashmap_free(m->units);
         hashmap_free(m->jobs);
         hashmap_free(m->watch_pids1);
@@ -1227,6 +1230,9 @@ int manager_startup(Manager *m, FILE *serialization, FDSet *fds) {
         /* Third, fire things up! */
         manager_coldplug(m);
 
+        /* Release any dynamic users no longer referenced */
+        dynamic_user_vacuum(m, true);
+
         if (serialization) {
                 assert(m->n_reloading > 0);
                 m->n_reloading--;
@@ -2403,6 +2409,10 @@ int manager_serialize(Manager *m, FILE *f, FDSet *fds, bool switching_root) {
 
         bus_track_serialize(m->subscribed, f);
 
+        r = dynamic_user_serialize(m, f, fds);
+        if (r < 0)
+                return r;
+
         fputc('\n', f);
 
         HASHMAP_FOREACH_KEY(u, t, m->units, i) {
@@ -2579,7 +2589,9 @@ int manager_deserialize(Manager *m, FILE *f, FDSet *fds) {
                                 m->kdbus_fd = fdset_remove(fds, fd);
                         }
 
-                } else {
+                } else if (startswith(l, "dynamic-user="))
+                        dynamic_user_deserialize_one(m, l + 13, fds);
+                else {
                         int k;
 
                         k = bus_track_deserialize_item(&m->deserialized_subscribed, l);
@@ -2660,6 +2672,7 @@ int manager_reload(Manager *m) {
         manager_clear_jobs_and_units(m);
         lookup_paths_flush_generator(&m->lookup_paths);
         lookup_paths_free(&m->lookup_paths);
+        dynamic_user_vacuum(m, false);
 
         q = lookup_paths_init(&m->lookup_paths, m->unit_file_scope, 0, NULL);
         if (q < 0 && r >= 0)
@@ -2696,6 +2709,9 @@ int manager_reload(Manager *m) {
         /* Third, fire things up! */
         manager_coldplug(m);
 
+        /* Release any dynamic users no longer referenced */
+        dynamic_user_vacuum(m, true);
+
         /* Sync current state of bus names with our set of listening units */
         if (m->api_bus)
                 manager_sync_bus_names(m, m->api_bus);
diff --git a/src/core/manager.h b/src/core/manager.h
index 6ed15c1a41..c681d5dc46 100644
--- a/src/core/manager.h
+++ b/src/core/manager.h
@@ -298,6 +298,9 @@ struct Manager {
         /* Used for processing polkit authorization responses */
         Hashmap *polkit_registry;
 
+        /* Dynamic users/groups, indexed by their name */
+        Hashmap *dynamic_users;
+
         /* When the user hits C-A-D more than 7 times per 2s, reboot immediately... */
         RateLimit ctrl_alt_del_ratelimit;
 
diff --git a/src/core/mount.c b/src/core/mount.c
index fda4d65d6f..db5cafcb11 100644
--- a/src/core/mount.c
+++ b/src/core/mount.c
@@ -245,6 +245,8 @@ static void mount_done(Unit *u) {
         exec_command_done_array(m->exec_command, _MOUNT_EXEC_COMMAND_MAX);
         m->control_command = NULL;
 
+        dynamic_creds_unref(&m->dynamic_creds);
+
         mount_unwatch_control_pid(m);
 
         m->timer_event_source = sd_event_source_unref(m->timer_event_source);
@@ -648,6 +650,9 @@ static int mount_coldplug(Unit *u) {
                         return r;
         }
 
+        if (!IN_SET(new_state, MOUNT_DEAD, MOUNT_FAILED))
+                (void) unit_setup_dynamic_creds(u);
+
         mount_set_state(m, new_state);
         return 0;
 }
@@ -716,6 +721,10 @@ static int mount_spawn(Mount *m, ExecCommand *c, pid_t *_pid) {
         if (r < 0)
                 return r;
 
+        r = unit_setup_dynamic_creds(UNIT(m));
+        if (r < 0)
+                return r;
+
         r = mount_arm_timer(m, usec_add(now(CLOCK_MONOTONIC), m->timeout_usec));
         if (r < 0)
                 return r;
@@ -732,6 +741,7 @@ static int mount_spawn(Mount *m, ExecCommand *c, pid_t *_pid) {
                        &m->exec_context,
                        &exec_params,
                        m->exec_runtime,
+                       &m->dynamic_creds,
                        &pid);
         if (r < 0)
                 return r;
@@ -752,12 +762,14 @@ static void mount_enter_dead(Mount *m, MountResult f) {
         if (f != MOUNT_SUCCESS)
                 m->result = f;
 
+        mount_set_state(m, m->result != MOUNT_SUCCESS ? MOUNT_FAILED : MOUNT_DEAD);
+
         exec_runtime_destroy(m->exec_runtime);
         m->exec_runtime = exec_runtime_unref(m->exec_runtime);
 
         exec_context_destroy_runtime_directory(&m->exec_context, manager_get_runtime_prefix(UNIT(m)->manager));
 
-        mount_set_state(m, m->result != MOUNT_SUCCESS ? MOUNT_FAILED : MOUNT_DEAD);
+        dynamic_creds_destroy(&m->dynamic_creds);
 }
 
 static void mount_enter_mounted(Mount *m, MountResult f) {
@@ -1817,6 +1829,7 @@ const UnitVTable mount_vtable = {
         .cgroup_context_offset = offsetof(Mount, cgroup_context),
         .kill_context_offset = offsetof(Mount, kill_context),
         .exec_runtime_offset = offsetof(Mount, exec_runtime),
+        .dynamic_creds_offset = offsetof(Mount, dynamic_creds),
 
         .sections =
                 "Unit\0"
diff --git a/src/core/mount.h b/src/core/mount.h
index da529c44f4..ac27b518cc 100644
--- a/src/core/mount.h
+++ b/src/core/mount.h
@@ -21,8 +21,8 @@
 
 typedef struct Mount Mount;
 
-#include "execute.h"
 #include "kill.h"
+#include "dynamic-user.h"
 
 typedef enum MountExecCommand {
         MOUNT_EXEC_MOUNT,
@@ -85,6 +85,7 @@ struct Mount {
         CGroupContext cgroup_context;
 
         ExecRuntime *exec_runtime;
+        DynamicCreds dynamic_creds;
 
         MountState state, deserialized_state;
 
diff --git a/src/core/service.c b/src/core/service.c
index afb198507b..4d59d78ecb 100644
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -322,6 +322,8 @@ static void service_done(Unit *u) {
         s->control_command = NULL;
         s->main_command = NULL;
 
+        dynamic_creds_unref(&s->dynamic_creds);
+
         exit_status_set_free(&s->restart_prevent_status);
         exit_status_set_free(&s->restart_force_status);
         exit_status_set_free(&s->success_status);
@@ -1030,6 +1032,9 @@ static int service_coldplug(Unit *u) {
         if (IN_SET(s->deserialized_state, SERVICE_START_POST, SERVICE_RUNNING, SERVICE_RELOAD))
                 service_start_watchdog(s);
 
+        if (!IN_SET(s->deserialized_state, SERVICE_DEAD, SERVICE_FAILED, SERVICE_AUTO_RESTART))
+                (void) unit_setup_dynamic_creds(u);
+
         service_set_state(s, s->deserialized_state);
         return 0;
 }
@@ -1184,6 +1189,10 @@ static int service_spawn(
         if (r < 0)
                 return r;
 
+        r = unit_setup_dynamic_creds(UNIT(s));
+        if (r < 0)
+                return r;
+
         if (pass_fds ||
             s->exec_context.std_input == EXEC_INPUT_SOCKET ||
             s->exec_context.std_output == EXEC_OUTPUT_SOCKET ||
@@ -1285,6 +1294,7 @@ static int service_spawn(
                        &s->exec_context,
                        &exec_params,
                        s->exec_runtime,
+                       &s->dynamic_creds,
                        &pid);
         if (r < 0)
                 return r;
@@ -1418,9 +1428,12 @@ static void service_enter_dead(Service *s, ServiceResult f, bool allow_restart)
         exec_runtime_destroy(s->exec_runtime);
         s->exec_runtime = exec_runtime_unref(s->exec_runtime);
 
-        /* Also, remove the runtime directory in */
+        /* Also, remove the runtime directory */
         exec_context_destroy_runtime_directory(&s->exec_context, manager_get_runtime_prefix(UNIT(s)->manager));
 
+        /* Release the user, and destroy it if we are the only remaining owner */
+        dynamic_creds_destroy(&s->dynamic_creds);
+
         /* Try to delete the pid file. At this point it will be
          * out-of-date, and some software might be confused by it, so
          * let's remove it. */
@@ -3323,6 +3336,7 @@ const UnitVTable service_vtable = {
         .cgroup_context_offset = offsetof(Service, cgroup_context),
         .kill_context_offset = offsetof(Service, kill_context),
         .exec_runtime_offset = offsetof(Service, exec_runtime),
+        .dynamic_creds_offset = offsetof(Service, dynamic_creds),
 
         .sections =
                 "Unit\0"
diff --git a/src/core/service.h b/src/core/service.h
index cfef375b03..8e56e1acb9 100644
--- a/src/core/service.h
+++ b/src/core/service.h
@@ -148,6 +148,7 @@ struct Service {
 
         /* Runtime data of the execution context */
         ExecRuntime *exec_runtime;
+        DynamicCreds dynamic_creds;
 
         pid_t main_pid, control_pid;
         int socket_fd;
diff --git a/src/core/socket.c b/src/core/socket.c
index e098055885..1ce41a1f07 100644
--- a/src/core/socket.c
+++ b/src/core/socket.c
@@ -150,6 +150,8 @@ static void socket_done(Unit *u) {
         exec_command_free_array(s->exec_command, _SOCKET_EXEC_COMMAND_MAX);
         s->control_command = NULL;
 
+        dynamic_creds_unref(&s->dynamic_creds);
+
         socket_unwatch_control_pid(s);
 
         unit_ref_unset(&s->service);
@@ -1602,6 +1604,9 @@ static int socket_coldplug(Unit *u) {
                         return r;
         }
 
+        if (!IN_SET(s->deserialized_state, SOCKET_DEAD, SOCKET_FAILED))
+                (void) unit_setup_dynamic_creds(u);
+
         socket_set_state(s, s->deserialized_state);
         return 0;
 }
@@ -1633,6 +1638,10 @@ static int socket_spawn(Socket *s, ExecCommand *c, pid_t *_pid) {
         if (r < 0)
                 return r;
 
+        r = unit_setup_dynamic_creds(UNIT(s));
+        if (r < 0)
+                return r;
+
         r = socket_arm_timer(s, usec_add(now(CLOCK_MONOTONIC), s->timeout_usec));
         if (r < 0)
                 return r;
@@ -1654,6 +1663,7 @@ static int socket_spawn(Socket *s, ExecCommand *c, pid_t *_pid) {
                        &s->exec_context,
                        &exec_params,
                        s->exec_runtime,
+                       &s->dynamic_creds,
                        &pid);
         if (r < 0)
                 return r;
@@ -1757,12 +1767,14 @@ static void socket_enter_dead(Socket *s, SocketResult f) {
         if (f != SOCKET_SUCCESS)
                 s->result = f;
 
+        socket_set_state(s, s->result != SOCKET_SUCCESS ? SOCKET_FAILED : SOCKET_DEAD);
+
         exec_runtime_destroy(s->exec_runtime);
         s->exec_runtime = exec_runtime_unref(s->exec_runtime);
 
         exec_context_destroy_runtime_directory(&s->exec_context, manager_get_runtime_prefix(UNIT(s)->manager));
 
-        socket_set_state(s, s->result != SOCKET_SUCCESS ? SOCKET_FAILED : SOCKET_DEAD);
+        dynamic_creds_destroy(&s->dynamic_creds);
 }
 
 static void socket_enter_signal(Socket *s, SocketState state, SocketResult f);
@@ -2930,6 +2942,7 @@ const UnitVTable socket_vtable = {
         .cgroup_context_offset = offsetof(Socket, cgroup_context),
         .kill_context_offset = offsetof(Socket, kill_context),
         .exec_runtime_offset = offsetof(Socket, exec_runtime),
+        .dynamic_creds_offset = offsetof(Socket, dynamic_creds),
 
         .sections =
                 "Unit\0"
diff --git a/src/core/socket.h b/src/core/socket.h
index 0f1ac69c6f..6c32d67bef 100644
--- a/src/core/socket.h
+++ b/src/core/socket.h
@@ -94,7 +94,9 @@ struct Socket {
         ExecContext exec_context;
         KillContext kill_context;
         CGroupContext cgroup_context;
+
         ExecRuntime *exec_runtime;
+        DynamicCreds dynamic_creds;
 
         /* For Accept=no sockets refers to the one service we'll
         activate. For Accept=yes sockets is either NULL, or filled
diff --git a/src/core/swap.c b/src/core/swap.c
index a532b15be8..66a318d01f 100644
--- a/src/core/swap.c
+++ b/src/core/swap.c
@@ -153,6 +153,8 @@ static void swap_done(Unit *u) {
         exec_command_done_array(s->exec_command, _SWAP_EXEC_COMMAND_MAX);
         s->control_command = NULL;
 
+        dynamic_creds_unref(&s->dynamic_creds);
+
         swap_unwatch_control_pid(s);
 
         s->timer_event_source = sd_event_source_unref(s->timer_event_source);
@@ -553,6 +555,9 @@ static int swap_coldplug(Unit *u) {
                         return r;
         }
 
+        if (!IN_SET(new_state, SWAP_DEAD, SWAP_FAILED))
+                (void) unit_setup_dynamic_creds(u);
+
         swap_set_state(s, new_state);
         return 0;
 }
@@ -628,6 +633,10 @@ static int swap_spawn(Swap *s, ExecCommand *c, pid_t *_pid) {
         if (r < 0)
                 goto fail;
 
+        r = unit_setup_dynamic_creds(UNIT(s));
+        if (r < 0)
+                return r;
+
         r = swap_arm_timer(s, usec_add(now(CLOCK_MONOTONIC), s->timeout_usec));
         if (r < 0)
                 goto fail;
@@ -644,6 +653,7 @@ static int swap_spawn(Swap *s, ExecCommand *c, pid_t *_pid) {
                        &s->exec_context,
                        &exec_params,
                        s->exec_runtime,
+                       &s->dynamic_creds,
                        &pid);
         if (r < 0)
                 goto fail;
@@ -668,12 +678,14 @@ static void swap_enter_dead(Swap *s, SwapResult f) {
         if (f != SWAP_SUCCESS)
                 s->result = f;
 
+        swap_set_state(s, s->result != SWAP_SUCCESS ? SWAP_FAILED : SWAP_DEAD);
+
         exec_runtime_destroy(s->exec_runtime);
         s->exec_runtime = exec_runtime_unref(s->exec_runtime);
 
         exec_context_destroy_runtime_directory(&s->exec_context, manager_get_runtime_prefix(UNIT(s)->manager));
 
-        swap_set_state(s, s->result != SWAP_SUCCESS ? SWAP_FAILED : SWAP_DEAD);
+        dynamic_creds_destroy(&s->dynamic_creds);
 }
 
 static void swap_enter_active(Swap *s, SwapResult f) {
@@ -1466,6 +1478,7 @@ const UnitVTable swap_vtable = {
         .cgroup_context_offset = offsetof(Swap, cgroup_context),
         .kill_context_offset = offsetof(Swap, kill_context),
         .exec_runtime_offset = offsetof(Swap, exec_runtime),
+        .dynamic_creds_offset = offsetof(Swap, dynamic_creds),
 
         .sections =
                 "Unit\0"
diff --git a/src/core/swap.h b/src/core/swap.h
index fbf66debdc..b0ef50f1e8 100644
--- a/src/core/swap.h
+++ b/src/core/swap.h
@@ -82,6 +82,7 @@ struct Swap {
         CGroupContext cgroup_context;
 
         ExecRuntime *exec_runtime;
+        DynamicCreds dynamic_creds;
 
         SwapState state, deserialized_state;
 
diff --git a/src/core/unit.c b/src/core/unit.c
index 4934a0e56f..ff7c562fba 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -3224,6 +3224,33 @@ void unit_ref_unset(UnitRef *ref) {
         ref->unit = NULL;
 }
 
+static int user_from_unit_name(Unit *u, char **ret) {
+
+        static const uint8_t hash_key[] = {
+                0x58, 0x1a, 0xaf, 0xe6, 0x28, 0x58, 0x4e, 0x96,
+                0xb4, 0x4e, 0xf5, 0x3b, 0x8c, 0x92, 0x07, 0xec
+        };
+
+        _cleanup_free_ char *n = NULL;
+        int r;
+
+        r = unit_name_to_prefix(u->id, &n);
+        if (r < 0)
+                return r;
+
+        if (valid_user_group_name(n)) {
+                *ret = n;
+                n = NULL;
+                return 0;
+        }
+
+        /* If we can't use the unit name as a user name, then let's hash it and use that */
+        if (asprintf(ret, "_du%016" PRIx64, siphash24(n, strlen(n), hash_key)) < 0)
+                return -ENOMEM;
+
+        return 0;
+}
+
 int unit_patch_contexts(Unit *u) {
         CGroupContext *cc;
         ExecContext *ec;
@@ -3268,6 +3295,22 @@ int unit_patch_contexts(Unit *u) {
 
                 if (ec->private_devices)
                         ec->capability_bounding_set &= ~(UINT64_C(1) << CAP_MKNOD);
+
+                if (ec->dynamic_user) {
+                        if (!ec->user) {
+                                r = user_from_unit_name(u, &ec->user);
+                                if (r < 0)
+                                        return r;
+                        }
+
+                        if (!ec->group) {
+                                ec->group = strdup(ec->user);
+                                if (!ec->group)
+                                        return -ENOMEM;
+                        }
+
+                        ec->private_tmp = true;
+                }
         }
 
         cc = unit_get_cgroup_context(u);
@@ -3776,6 +3819,26 @@ int unit_setup_exec_runtime(Unit *u) {
         return exec_runtime_make(rt, unit_get_exec_context(u), u->id);
 }
 
+int unit_setup_dynamic_creds(Unit *u) {
+        ExecContext *ec;
+        DynamicCreds *dcreds;
+        size_t offset;
+
+        assert(u);
+
+        offset = UNIT_VTABLE(u)->dynamic_creds_offset;
+        assert(offset > 0);
+        dcreds = (DynamicCreds*) ((uint8_t*) u + offset);
+
+        ec = unit_get_exec_context(u);
+        assert(ec);
+
+        if (!ec->dynamic_user)
+                return 0;
+
+        return dynamic_creds_acquire(dcreds, u->manager, ec->user, ec->group);
+}
+
 bool unit_type_supported(UnitType t) {
         if (_unlikely_(t < 0))
                 return false;
diff --git a/src/core/unit.h b/src/core/unit.h
index 1eabfa51e2..47eb8d50a6 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -291,6 +291,10 @@ struct UnitVTable {
          * that */
         size_t exec_runtime_offset;
 
+        /* If greater than 0, the offset into the object where the pointer to DynamicCreds is found, if the unit type
+         * has that. */
+        size_t dynamic_creds_offset;
+
         /* The name of the configuration file section with the private settings of this unit */
         const char *private_section;
 
@@ -589,6 +593,7 @@ CGroupContext *unit_get_cgroup_context(Unit *u) _pure_;
 ExecRuntime *unit_get_exec_runtime(Unit *u) _pure_;
 
 int unit_setup_exec_runtime(Unit *u);
+int unit_setup_dynamic_creds(Unit *u);
 
 int unit_write_drop_in(Unit *u, UnitSetPropertiesMode mode, const char *name, const char *data);
 int unit_write_drop_in_format(Unit *u, UnitSetPropertiesMode mode, const char *name, const char *format, ...) _printf_(4,5);