Fix SELinux labels in cgroup filesystem root directory (#7496)

When using SELinux with legacy cgroups the tmpfs on /sys/fs/cgroup is by
default labelled as tmpfs_t. This label is also inherited by the "cpu"
and "cpuacct" symbolic links. Unfortunately the policy expects them to
be labelled as cgroup_t, which is used for all the actual cgroup
filesystems. Failure to do so results in a stream of denials.

This state cannot be fixed reliably when the cgroup filesystem structure
is set-up as the SELinux policy is not yet loaded at this
moment. It also cannot be fixed later as the root of the cgroup
filesystem is remounted read-only. In order to fix it the root of the
cgroup filesystem needs to be temporary remounted read-write, relabelled
and remounted back read-only.

diff --git a/src/core/mount-setup.c b/src/core/mount-setup.c
index d5c0fcd..7171d8f 100644 (file)
--- a/src/core/mount-setup.c
+++ b/src/core/mount-setup.c
@@ -390,9 +390,19 @@ int mount_setup(bool loaded_policy) {
                 nftw("/dev/shm", nftw_cb, 64, FTW_MOUNT|FTW_PHYS|FTW_ACTIONRETVAL);
                 nftw("/run", nftw_cb, 64, FTW_MOUNT|FTW_PHYS|FTW_ACTIONRETVAL);
 
+                /* Temporarily remount the root cgroup filesystem to give it a proper label. */
+                r = cg_all_unified();
+                if (r == 0) {
+                        (void) mount(NULL, "/sys/fs/cgroup", NULL, MS_REMOUNT, NULL);
+                        label_fix("/sys/fs/cgroup", false, false);
+                        nftw("/sys/fs/cgroup", nftw_cb, 64, FTW_MOUNT|FTW_PHYS|FTW_ACTIONRETVAL);
+                        (void) mount(NULL, "/sys/fs/cgroup", NULL, MS_REMOUNT|MS_RDONLY, NULL);
+                } else if (r < 0)
+                        return log_error_errno(r, "Failed to determine whether we are in all unified mode: %m");
+
                 after_relabel = now(CLOCK_MONOTONIC);
 
-                log_info("Relabelled /dev and /run in %s.",
+                log_info("Relabelled /dev, /run and /sys/fs/cgroup in %s.",
                          format_timespan(timespan, sizeof(timespan), after_relabel - before_relabel, 0));
         }
 #endif