Re: [PATCH] xfs: fix use-after-free on CIL context on shutdown

["yukuai (C)" <yukuai3@huawei.com>]

On 2020/6/11 10:45, Dave Chinner wrote:
> 
> From: Dave Chinner <dchinner@redhat.com>
> 
> xlog_wait() on the CIL context can reference a freed context if the
> waiter doesn't get scheduled before the CIL context is freed. This
> can happen when a task is on the hard throttle and the CIL push
> aborts due to a shutdown. This was detected by generic/019:
> 
> thread 1			thread 2
> 
> __xfs_trans_commit
>   xfs_log_commit_cil
>    <CIL size over hard throttle limit>
>    xlog_wait
>     schedule
> 				xlog_cil_push_work
> 				wake_up_all
> 				<shutdown aborts commit>
> 				xlog_cil_committed
> 				kmem_free
> 
>     remove_wait_queue
>      spin_lock_irqsave --> UAF
> 
> Fix it by moving the wait queue to the CIL rather than keeping it in
> in the CIL context that gets freed on push completion. Because the
> wait queue is now independent of the CIL context and we might have
> multiple contexts in flight at once, only wake the waiters on the
> push throttle when the context we are pushing is over the hard
> throttle size threshold.

Hi, Dave,

How do you think about the following fix:

1. use autoremove_wake_func(), and remove remove_wait_queue() to
avoid UAF.
2. add finish_wait().

@@ -576,12 +576,13 @@ xlog_wait(
                 __releases(lock)
  {
         DECLARE_WAITQUEUE(wait, current);
+       wait.func = autoremove_wake_function;

         add_wait_queue_exclusive(wq, &wait);
         __set_current_state(TASK_UNINTERRUPTIBLE);
         spin_unlock(lock);
         schedule();
-       remove_wait_queue(wq, &wait);
+       finish_wait(wq, &wait);
  }

Best regards!
Yu Kuai