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: Nyangerkun <yangerkun@huawei.com> Signed-off-by: NJeff Layton <jlayton@kernel.org>
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