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: d7e7b9af104c ("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.com
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230313221231.272498-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/super.c b/fs/super.c
index 84332d5..04bc62a 100644 (file)
--- 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;