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

[ Upstream commit 6d390e4b5d48ec03bb87e63cf0a2bff5f4e116da ]

'16306a61d3b7 ("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´╝Ü

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(will insert
				    our fl_blocked_member list
				    to flock a's fl_blocked_requests)
flock syscall(unlock)
	b->fl_blocker = NULL)
                                   break by a signal
				    b->fl_blocker == NULL &&
	                            success, return directly
				 locks_free_lock b
	trigger UAF

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

Cc: stable@vger.kernel.org
Fixes: 16306a61d3b7 ("fs/locks: always delete_block after waiting.")
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
diff --git a/fs/locks.c b/fs/locks.c
index 44b6da0..426b55d 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -753,20 +753,6 @@
 	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;
 	if (waiter->fl_blocker)
 		status = 0;