ksmbd: fix use-after-free in smb2_lock

commit 84d2d16 upstream.

If smb_lock->zero_len has value, ->llist of smb_lock is not delete and
flock is old one. It will cause use-after-free on error handling
routine.

Cc: stable@vger.kernel.org
Reported-by: Norbert Szetei <norbert@doyensec.com>
Tested-by: Norbert Szetei <norbert@doyensec.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: namjaejeon

fs/smb/server/smb2pdu.c

@@ -7441,13 +7441,13 @@ int smb2_lock(struct ksmbd_work *work)
 		}
 
 no_check_cl:
+		flock = smb_lock->fl;
+		list_del(&smb_lock->llist);
+
 		if (smb_lock->zero_len) {
 			err = 0;
 			goto skip;
 		}
-
-		flock = smb_lock->fl;
-		list_del(&smb_lock->llist);
 retry:
 		rc = vfs_lock_file(filp, smb_lock->cmd, flock, NULL);
 skip: