/***
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"
/* 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) % (DYNAMIC_UID_MAX - DYNAMIC_UID_MIN + 1)) + DYNAMIC_UID_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);
return mfree(d);
}
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 make_uid_symlinks(uid_t uid, const char *name, bool b) {
char path1[strlen("/run/systemd/dynamic-uid/direct:") + DECIMAL_STR_MAX(uid_t) + 1];
const char *path2;
int r = 0, k;
/* Add direct additional symlinks for direct lookups of dynamic UIDs and their names by userspace code. The
* only reason we have this is because dbus-daemon cannot use D-Bus for resolving users and groups (since it
* would be its own client then). We hence keep these world-readable symlinks in place, so that the
* unprivileged dbus user can read the mappings when it needs them via these symlinks instead of having to go
* via the bus. Ideally, we'd use the lock files we keep for this anyway, but we can't since we use BSD locks
* on them and as those may be taken by any user with read access we can't make them world-readable. */
xsprintf(path1, "/run/systemd/dynamic-uid/direct:" UID_FMT, uid);
if (unlink(path1) < 0 && errno != ENOENT)
r = -errno;
if (b && symlink(name, path1) < 0) {
k = log_warning_errno(errno, "Failed to symlink \"%s\": %m", path1);
if (r == 0)
r = k;
}
path2 = strjoina("/run/systemd/dynamic-uid/direct:", name);
if (unlink(path2) < 0 && errno != ENOENT) {
k = -errno;
if (r == 0)
r = k;
}
if (b && symlink(path1 + strlen("/run/systemd/dynamic-uid/direct:"), path2) < 0) {
k = log_warning_errno(errno, "Failed to symlink \"%s\": %m", path2);
if (r == 0)
r = k;
}
return r;
}
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 (!uid_is_dynamic(candidate))
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 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);
(void) make_uid_symlinks(candidate, name, true); /* also add direct lookup symlinks */
*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, const char *name) {
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(lock_path);
(void) make_uid_symlinks(uid, name, false); /* remove direct lookup symlinks */
}
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, d->name);
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, d->name);
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, d->name);
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, d->name);
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 name. */
if (!uid_is_dynamic(uid))
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);
}