core: dont't remount /sys/fs/cgroup for relabel if not needed (#8595)

The initial fix for relabelling the cgroup filesystem for
SELinux delivered in commit 8739f23e3 was based on the assumption that
the cgroup filesystem is already populated once mount_setup() is
executed, which was true for my system. What I wasn't aware is that this
is the case only when another instance of systemd was running before
this one, which can happen if systemd is used in the initrd (for ex. by
dracut).

In case of a clean systemd start-up the cgroup filesystem is actually
being populated after mount_setup() and does not need relabelling as at
that moment the SELinux policy is already loaded. Since however the root
cgroup filesystem was remounted read-only in the meantime this operation
will now fail.

To fix this check for the filesystem mount flags before relabelling and
only remount ro->rw->ro if necessary and leave the filesystem read-write
otherwise.

Fixes #7901.

1 files changed, 33 insertions, 9 deletions

diff --git a/src/core/mount-setup.c b/src/core/mount-setup.c
index 0fe794d2af..a0b5bff5a7 100644
--- a/src/core/mount-setup.c
+++ b/src/core/mount-setup.c
@@ -22,6 +22,7 @@
 #include <ftw.h>
 #include <stdlib.h>
 #include <sys/mount.h>
+#include <sys/statvfs.h>
 #include <unistd.h>
 
 #include "alloc-util.h"
@@ -385,6 +386,35 @@ static int nftw_cb(
 
         return FTW_CONTINUE;
 };
+
+static int relabel_cgroup_filesystems(void) {
+        int r;
+        struct statfs st;
+
+        r = cg_all_unified();
+        if (r == 0) {
+                /* Temporarily remount the root cgroup filesystem to give it a proper label. Do this
+                   only when the filesystem has been already populated by a previous instance of systemd
+                   running from initrd. Otherwise don't remount anything and leave the filesystem read-write
+                   for the cgroup filesystems to be mounted inside. */
+                r = statfs("/sys/fs/cgroup", &st);
+                if (r < 0) {
+                        return log_error_errno(errno, "Failed to determine mount flags for /sys/fs/cgroup: %m");
+                }
+
+                if (st.f_flags & ST_RDONLY)
+                        (void) mount(NULL, "/sys/fs/cgroup", NULL, MS_REMOUNT, NULL);
+
+                (void) label_fix("/sys/fs/cgroup", 0);
+                nftw("/sys/fs/cgroup", nftw_cb, 64, FTW_MOUNT|FTW_PHYS|FTW_ACTIONRETVAL);
+
+                if (st.f_flags & ST_RDONLY)
+                        (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");
+
+        return 0;
+}
 #endif
 
 int mount_setup(bool loaded_policy) {
@@ -409,15 +439,9 @@ 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);
-                        (void) label_fix("/sys/fs/cgroup", 0);
-                        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");
+                r = relabel_cgroup_filesystems();
+                if (r < 0)
+                        return r;
 
                 after_relabel = now(CLOCK_MONOTONIC);