mnt: Fix fs_fully_visible to verify the root directory is visible

commit 7e96c1b0 upstream.

This fixes a dumb bug in fs_fully_visible that allows proc or sys to
be mounted if there is a bind mount of part of /proc/ or /sys/ visible.
Reported-by: Eric Windisch <ewindisch@docker.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/namespace.c

@@ -3038,6 +3038,12 @@ bool fs_fully_visible(struct file_system_type *type)
 		if (mnt->mnt.mnt_sb->s_type != type)
 			continue;
 
+		/* This mount is not fully visible if it's root directory
+		 * is not the root directory of the filesystem.
+		 */
+		if (mnt->mnt.mnt_root != mnt->mnt.mnt_sb->s_root)
+			continue;
+
 		/* This mount is not fully visible if there are any child mounts
 		 * that cover anything except for empty directories.
 		 */