fscrypt: destroy keyring after security_sb_delete()

fscrypt_destroy_keyring() must be called after all potentially-encrypted
inodes were evicted; otherwise it cannot safely destroy the keyring.
Since inodes that are in-use by the Landlock LSM don't get evicted until
security_sb_delete(), this means that fscrypt_destroy_keyring() must be
called *after* security_sb_delete().

This fixes a WARN_ON followed by a NULL dereference, only possible if
Landlock was being used on encrypted files.

Fixes: d7e7b9af ("fscrypt: stop using keyrings subsystem for fscrypt_master_key")
Cc: stable@vger.kernel.org
Reported-by: syzbot+93e495f6a4f748827c88@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/00000000000044651705f6ca1e30@google.comReviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230313221231.272498-2-ebiggers@kernel.orgSigned-off-by: Eric Biggers <ebiggers@google.com>

fs/super.c

@@ -475,13 +475,22 @@ void generic_shutdown_super(struct super_block *sb)
 
 		cgroup_writeback_umount();
 
-		/* evict all inodes with zero refcount */
+		/* Evict all inodes with zero refcount. */
 		evict_inodes(sb);
-		/* only nonzero refcount inodes can have marks */
+
+		/*
+		 * Clean up and evict any inodes that still have references due
+		 * to fsnotify or the security policy.
+		 */
 		fsnotify_sb_delete(sb);
-		fscrypt_destroy_keyring(sb);
 		security_sb_delete(sb);
 
+		/*
+		 * Now that all potentially-encrypted inodes have been evicted,
+		 * the fscrypt keyring can be destroyed.
+		 */
+		fscrypt_destroy_keyring(sb);
+
 		if (sb->s_dio_done_wq) {
 			destroy_workqueue(sb->s_dio_done_wq);
 			sb->s_dio_done_wq = NULL;