[PATCH v2] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Siddh Raman Pant]

On Sun, 24 Jul 2022 10:54:26 +0530  Hillf Danton <hdanton at sina.com> wrote:
> According to sysbot's report [1], what is proposed fails to fix the
> reported uaf in the scenario below because of no wqueue->lock acquired
> in the post path. Despite of defunct checked, it is checked after taking
> pipe's wait lock - a bit too late.
> 
> Hillf
> 
> 	post_one_notification	watch_queue_clear	
> 	===			===
> 	pipe = wqueue->pipe;
> 	if (!pipe)
> 		return false;
> 				spin_lock_bh(&wqueue->lock);
> 				wqueue->pipe = NULL;
> 				spin_lock_bh(&wqueue->lock);
> 
> 				pipe is freed
> 
> 	spin_lock_irq(&pipe->rd_wait.lock); // uaf reported
> 
> 	if (wqueue->defunct)
> 		goto out;
> 
> [1] https://lore.kernel.org/lkml/000000000000aa07b205daba6d49@google.com/
> 

Before post_one_notification(), in __post_watch_notification(), on line 228
wqueue->lock is acquired (using lock_wqueue()).

The comment on post_one_notification() also tells that it should be called
with wqueue->lock held. So we do acquire the lock in the post path.

The pipe is freed after the execution of watch_queue_clear() in
free_pipe_info().

A side note: Your emails don't seem to pop up on lore's LKML archive. Maybe
check if the email isn't being blocked?

Thanks,
Siddh