[PATCH 1/3] fscrypt: destroy keyring after security_sb_delete()

[Eric Biggers]

From: Eric Biggers <ebiggers at google.com>

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: d7e7b9af104c ("fscrypt: stop using keyrings subsystem for fscrypt_master_key")
Cc: stable at vger.kernel.org
Reported-by: syzbot+93e495f6a4f748827c88 at syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/00000000000044651705f6ca1e30@google.com
Signed-off-by: Eric Biggers <ebiggers at google.com>
---
 fs/super.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 84332d5cb817a..04bc62ab7dfea 100644
--- a/fs/super.c
+++ b/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;
-- 
2.39.2