apparmor: Fix move_mount mediation by detecting if source is detached

Prevent move_mount from applying the attach_disconnected flag
to move_mount(). This prevents detached mounts from appearing
as / when applying mount mediation, which is not only incorrect
but could result in bad policy being generated.

Basic mount rules like
  allow mount,
  allow mount options=(move) -> /target/,

will allow detached mounts, allowing older policy to continue
to function. New policy gains the ability to specify `detached` as
a source option
  allow mount detached -> /target/,

In addition make sure support of move_mount is advertised as
a feature to userspace so that applications that generate policy
can respond to the addition.

Note: this fixes mediation of move_mount when a detached mount is used,
      it does not fix the broader regression of apparmor mediation of
      mounts under the new mount api.

Link: https://lore.kernel.org/all/68c166b8-5b4d-4612-8042-1dee3334385b@leemhuis.info/T/#mb35fdde37f999f08f0b02d58dc1bf4e6b65b8da2
Fixes: 157a3537 ("apparmor: Fix regression in mount mediation")
Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/apparmorfs.c

@@ -2373,6 +2373,7 @@ static struct aa_sfs_entry aa_sfs_entry_policy[] = {
 
 static struct aa_sfs_entry aa_sfs_entry_mount[] = {
 	AA_SFS_FILE_STRING("mask", "mount umount pivot_root"),
+	AA_SFS_FILE_STRING("move_mount", "detached"),
 	{ }
 };
 

security/apparmor/mount.c

@@ -499,6 +499,10 @@ int aa_move_mount(const struct cred *subj_cred,
 	error = -ENOMEM;
 	if (!to_buffer || !from_buffer)
 		goto out;
+
+	if (!our_mnt(from_path->mnt))
+		/* moving a mount detached from the namespace */
+		from_path = NULL;
 	error = fn_for_each_confined(label, profile,
 			match_mnt(subj_cred, profile, to_path, to_buffer,
 				  from_path, from_buffer,