locks: fix a potential use-after-free problem when wakeup a waiter

'16306a61 ("fs/locks: always delete_block after waiting.")' add the
logic to check waiter->fl_blocker without blocked_lock_lock. And it will
trigger a UAF when we try to wakeup some waiter：

Thread 1 has create a write flock a on file, and now thread 2 try to
unlock and delete flock a, thread 3 try to add flock b on the same file.

Thread2                         Thread3
                                flock syscall(create flock b)
	                        ...flock_lock_inode_wait
				    flock_lock_inode(will insert
				    our fl_blocked_member list
				    to flock a's fl_blocked_requests)
				   sleep
flock syscall(unlock)
...flock_lock_inode_wait
    locks_delete_lock_ctx
    ...__locks_wake_up_blocks
        __locks_delete_blocks(
	b->fl_blocker = NULL)
	...
                                   break by a signal
				   locks_delete_block
				    b->fl_blocker == NULL &&
				    list_empty(&b->fl_blocked_requests)
	                            success, return directly
				 locks_free_lock b
	wake_up(&b->fl_waiter)
	trigger UAF

Fix it by remove this logic, and this patch may also fix CVE-2019-19769.

Cc: stable@vger.kernel.org
Fixes: 16306a61 ("fs/locks: always delete_block after waiting.")
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>

fs/locks.c

@@ -753,20 +753,6 @@ int locks_delete_block(struct file_lock *waiter)
 {
 	int status = -ENOENT;
 
-	/*
-	 * If fl_blocker is NULL, it won't be set again as this thread
-	 * "owns" the lock and is the only one that might try to claim
-	 * the lock.  So it is safe to test fl_blocker locklessly.
-	 * Also if fl_blocker is NULL, this waiter is not listed on
-	 * fl_blocked_requests for some lock, so no other request can
-	 * be added to the list of fl_blocked_requests for this
-	 * request.  So if fl_blocker is NULL, it is safe to
-	 * locklessly check if fl_blocked_requests is empty.  If both
-	 * of these checks succeed, there is no need to take the lock.
-	 */
-	if (waiter->fl_blocker == NULL &&
-	    list_empty(&waiter->fl_blocked_requests))
-		return status;
 	spin_lock(&blocked_lock_lock);
 	if (waiter->fl_blocker)
 		status = 0;