nspawn: skip /sys-as-tmpfs if we don't use private-network

Since v3.11/7dc5dbc ("sysfs: Restrict mounting sysfs"), the kernel
doesn't allow mounting sysfs if you don't have CAP_SYS_ADMIN rights over
the network namespace.

So the mounting /sys as a tmpfs code introduced in
d8fc6a000fe21b0c1ba27fbfed8b42d00b349a4b doesn't work with user
namespaces if we don't use private-net. The reason is that we mount
sysfs inside the container and we're in the network namespace of the host
but we don't have CAP_SYS_ADMIN over that namespace.

To fix that, we mount /sys as a sysfs (instead of tmpfs) if we don't use
private network and ignore the /sys-as-a-tmpfs code if we find that /sys
is already mounted as sysfs.

Fixes #1555

3 files changed, 33 insertions, 15 deletions

diff --git a/src/nspawn/nspawn-mount.c b/src/nspawn/nspawn-mount.c
index 6c8b1d7a26..65bcb68242 100644
--- a/src/nspawn/nspawn-mount.c
+++ b/src/nspawn/nspawn-mount.c
@@ -20,6 +20,7 @@
 ***/
 
 #include <sys/mount.h>
+#include <linux/magic.h>
 
 #include "util.h"
 #include "rm-rf.h"
@@ -218,8 +219,19 @@ static int tmpfs_patch_options(
 
 int mount_sysfs(const char *dest) {
         const char *full, *top, *x;
+        int r;
 
         top = prefix_roota(dest, "/sys");
+        r = path_check_fstype(top, SYSFS_MAGIC);
+        if (r < 0)
+                return log_error_errno(r, "Failed to determine filesystem type of %s: %m", top);
+        /* /sys might already be mounted as sysfs by the outer child in the
+         * !netns case. In this case, it's all good. Don't touch it because we
+         * don't have the right to do so, see https://github.com/systemd/systemd/issues/1555.
+         */
+        if (r > 0)
+                return 0;
+
         full = prefix_roota(top, "/full");
 
         (void) mkdir(full, 0755);
@@ -264,6 +276,7 @@ int mount_sysfs(const char *dest) {
 
 int mount_all(const char *dest,
               bool use_userns, bool in_userns,
+              bool use_netns,
               uid_t uid_shift, uid_t uid_range,
               const char *selinux_apifs_context) {
 
@@ -274,21 +287,23 @@ int mount_all(const char *dest,
                 const char *options;
                 unsigned long flags;
                 bool fatal;
-                bool userns;
+                bool in_userns;
+                bool use_netns;
         } MountPoint;
 
         static const MountPoint mount_table[] = {
-                { "proc",      "/proc",          "proc",   NULL,        MS_NOSUID|MS_NOEXEC|MS_NODEV,                              true,  true  },
-                { "/proc/sys", "/proc/sys",      NULL,     NULL,        MS_BIND,                                                   true,  true  },   /* Bind mount first */
-                { NULL,        "/proc/sys",      NULL,     NULL,        MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT, true,  true  },   /* Then, make it r/o */
-                { "tmpfs",     "/sys",           "tmpfs",  "mode=755",  MS_NOSUID|MS_NOEXEC|MS_NODEV,                              true,  false },
-                { "tmpfs",     "/dev",           "tmpfs",  "mode=755",  MS_NOSUID|MS_STRICTATIME,                                  true,  false },
-                { "tmpfs",     "/dev/shm",       "tmpfs",  "mode=1777", MS_NOSUID|MS_NODEV|MS_STRICTATIME,                         true,  false },
-                { "tmpfs",     "/run",           "tmpfs",  "mode=755",  MS_NOSUID|MS_NODEV|MS_STRICTATIME,                         true,  false },
-                { "tmpfs",     "/tmp",           "tmpfs",  "mode=1777", MS_STRICTATIME,                                            true,  false },
+                { "proc",      "/proc",          "proc",   NULL,        MS_NOSUID|MS_NOEXEC|MS_NODEV,                              true,  true, false  },
+                { "/proc/sys", "/proc/sys",      NULL,     NULL,        MS_BIND,                                                   true,  true, false  },   /* Bind mount first */
+                { NULL,        "/proc/sys",      NULL,     NULL,        MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT, true,  true, false  },   /* Then, make it r/o */
+                { "tmpfs",     "/sys",           "tmpfs",  "mode=755",  MS_NOSUID|MS_NOEXEC|MS_NODEV,                              true,  false, true },
+                { "sysfs",     "/sys",           "sysfs",  NULL,        MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV,                    true,  false, false },
+                { "tmpfs",     "/dev",           "tmpfs",  "mode=755",  MS_NOSUID|MS_STRICTATIME,                                  true,  false, false },
+                { "tmpfs",     "/dev/shm",       "tmpfs",  "mode=1777", MS_NOSUID|MS_NODEV|MS_STRICTATIME,                         true,  false, false },
+                { "tmpfs",     "/run",           "tmpfs",  "mode=755",  MS_NOSUID|MS_NODEV|MS_STRICTATIME,                         true,  false, false },
+                { "tmpfs",     "/tmp",           "tmpfs",  "mode=1777", MS_STRICTATIME,                                            true,  false, false },
 #ifdef HAVE_SELINUX
-                { "/sys/fs/selinux", "/sys/fs/selinux", NULL, NULL,     MS_BIND,                                                   false, false },  /* Bind mount first */
-                { NULL,              "/sys/fs/selinux", NULL, NULL,     MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT, false, false },  /* Then, make it r/o */
+                { "/sys/fs/selinux", "/sys/fs/selinux", NULL, NULL,     MS_BIND,                                                   false, false, false },  /* Bind mount first */
+                { NULL,              "/sys/fs/selinux", NULL, NULL,     MS_BIND|MS_RDONLY|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_REMOUNT, false, false, false },  /* Then, make it r/o */
 #endif
         };
 
@@ -299,7 +314,10 @@ int mount_all(const char *dest,
                 _cleanup_free_ char *where = NULL, *options = NULL;
                 const char *o;
 
-                if (in_userns != mount_table[k].userns)
+                if (in_userns != mount_table[k].in_userns)
+                        continue;
+
+                if (!use_netns && mount_table[k].use_netns)
                         continue;
 
                 where = prefix_root(dest, mount_table[k].where);
diff --git a/src/nspawn/nspawn-mount.h b/src/nspawn/nspawn-mount.h
index 54cab87665..bdab23bcca 100644
--- a/src/nspawn/nspawn-mount.h
+++ b/src/nspawn/nspawn-mount.h
@@ -57,7 +57,7 @@ int tmpfs_mount_parse(CustomMount **l, unsigned *n, const char *s);
 
 int custom_mount_compare(const void *a, const void *b);
 
-int mount_all(const char *dest, bool use_userns, bool in_userns, uid_t uid_shift, uid_t uid_range, const char *selinux_apifs_context);
+int mount_all(const char *dest, bool use_userns, bool in_userns, bool use_netns, uid_t uid_shift, uid_t uid_range, const char *selinux_apifs_context);
 int mount_sysfs(const char *dest);
 
 int mount_cgroups(const char *dest, bool unified_requested, bool userns, uid_t uid_shift, uid_t uid_range, const char *selinux_apifs_context);
diff --git a/src/nspawn/nspawn.c b/src/nspawn/nspawn.c
index ab93f98df4..fca2b72edd 100644
--- a/src/nspawn/nspawn.c
+++ b/src/nspawn/nspawn.c
@@ -2450,7 +2450,7 @@ static int inner_child(
                 }
         }
 
-        r = mount_all(NULL, arg_userns, true, arg_uid_shift, arg_uid_range, arg_selinux_apifs_context);
+        r = mount_all(NULL, arg_userns, true, arg_uid_shift, arg_private_network, arg_uid_range, arg_selinux_apifs_context);
         if (r < 0)
                 return r;
 
@@ -2705,7 +2705,7 @@ static int outer_child(
                         return log_error_errno(r, "Failed to make tree read-only: %m");
         }
 
-        r = mount_all(directory, arg_userns, false, arg_uid_shift, arg_uid_range, arg_selinux_apifs_context);
+        r = mount_all(directory, arg_userns, false, arg_private_network, arg_uid_shift, arg_uid_range, arg_selinux_apifs_context);
         if (r < 0)
                 return r;