mnt: Move the test for MNT_LOCK_READONLY from change_mount_flags into do_remount

There are no races as locked mount flags are guaranteed to never change.

Moving the test into do_remount makes it more visible, and ensures all
filesystem remounts pass the MNT_LOCK_READONLY permission check.  This
second case is not an issue today as filesystem remounts are guarded
by capable(CAP_DAC_ADMIN) and thus will always fail in less privileged
mount namespaces, but it could become an issue in the future.

Cc: stable@vger.kernel.org
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/namespace.c

@@ -1896,9 +1896,6 @@ static int change_mount_flags(struct vfsmount *mnt, int ms_flags)
 	if (readonly_request == __mnt_is_readonly(mnt))
 		return 0;
 
-	if (mnt->mnt_flags & MNT_LOCK_READONLY)
-		return -EPERM;
-
 	if (readonly_request)
 		error = mnt_make_readonly(real_mount(mnt));
 	else
@@ -1924,6 +1921,16 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
 	if (path->dentry != path->mnt->mnt_root)
 		return -EINVAL;
 
+	/* Don't allow changing of locked mnt flags.
+	 *
+	 * No locks need to be held here while testing the various
+	 * MNT_LOCK flags because those flags can never be cleared
+	 * once they are set.
+	 */
+	if ((mnt->mnt.mnt_flags & MNT_LOCK_READONLY) &&
+	    !(mnt_flags & MNT_READONLY)) {
+		return -EPERM;
+	}
 	err = security_sb_remount(sb, data);
 	if (err)
 		return err;