On Wed, Aug 03, 2022 at 09:26:04AM +0530, Siddh Raman Pant wrote:

On Wed, 03 Aug 2022 06:38:36 +0530  Eric Biggers [off-list ref] wrote:

quoted

On Thu, Jul 28, 2022 at 09:21:21PM +0530, Siddh Raman Pant wrote:

quoted

If not done, a reference to a freed pipe remains in the watch_queue,
as this function is called before freeing a pipe in free_pipe_info()
(see line 834 of fs/pipe.c).

This causes a UAF when post_one_notification() tries to access the pipe
on a key update, which is reported by syzbot.

We also need to use READ_ONCE() in post_one_notification() to prevent the
compiler from optimising and loading a non-NULL value from wqueue->pipe.

Didn't this already get fixed by the following commit?

    commit 353f7988dd8413c47718f7ca79c030b6fb62cfe5
    Author: Linus Torvalds [off-list ref]
    Date:   Tue Jul 19 11:09:01 2022 -0700

        watchqueue: make sure to serialize 'wqueue->defunct' properly

With that, post_one_notification() only runs while the watch_queue is locked and
not "defunct".  So it's guaranteed that the pipe still exists.  Any concurrent
free_pipe_info() waits for the watch_queue to be unlocked in watch_queue_clear()
before proceeding to free the pipe.  So where is there still a bug?

It doesn't fix the dangling pointer to the freed pipe in the watch_queue, which
had caused this crash.

Under what circumstances is the pipe pointer still being dereferenced after the
pipe has been freed?  I don't see how it can be; see my explanation above.

- Eric