From 6d390e4b5d48ec03bb87e63cf0a2bff5f4e116da Mon Sep 17 00:00:00 2001 From: yangerkun Date: Wed, 4 Mar 2020 15:25:56 +0800 Subject: [PATCH] locks: fix a potential use-after-free problem when wakeup a waiter MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit '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： 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: 16306a61d3b7 ("fs/locks: always delete_block after waiting.") Signed-off-by: yangerkun Signed-off-by: Jeff Layton --- fs/locks.c | 14 -------------- 1 file changed, 14 deletions(-) diff --git a/fs/locks.c b/fs/locks.c index 44b6da032842..426b55d333d5 100644 --- a/fs/locks.c +++ b/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;