[PATCH v3] kernel/watch_queue: Make pipe NULL while clearing watch_queue

[Eric Biggers]

On Thu, Jul 28, 2022 at 09:21:21PM +0530, Siddh Raman Pant wrote:
> 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 <torvalds at linux-foundation.org>
	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?

> 
> Bug report: https://syzkaller.appspot.com/bug?id=1870dd7791ba05f2ea7f47f7cbdde701173973fc
> Reported-and-tested-by: syzbot+c70d87ac1d001f29a058 at syzkaller.appspotmail.com

If this actually does fix something, then it's mixing Fixes and Cc stable tags.

> diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
> index bb9962b33f95..617425e34252 100644
> --- a/kernel/watch_queue.c
> +++ b/kernel/watch_queue.c
> @@ -99,7 +99,7 @@ static bool post_one_notification(struct watch_queue *wqueue,
>  				  struct watch_notification *n)
>  {
>  	void *p;
> -	struct pipe_inode_info *pipe = wqueue->pipe;
> +	struct pipe_inode_info *pipe = READ_ONCE(wqueue->pipe);
>  	struct pipe_buffer *buf;
>  	struct page *page;
>  	unsigned int head, tail, mask, note, offset, len;
> @@ -637,6 +637,12 @@ void watch_queue_clear(struct watch_queue *wqueue)
>  		spin_lock_bh(&wqueue->lock);
>  	}
>  
> +	/* Clearing the watch queue, so we should clean the associated pipe. */
> +	if (wqueue->pipe) {
> +		wqueue->pipe->watch_queue = NULL;
> +		wqueue->pipe = NULL;
> +	}
> +
>  	spin_unlock_bh(&wqueue->lock);
>  	rcu_read_unlock();
>  }

And this is clearly the wrong fix anyway, since it makes the call to
put_watch_queue() in free_pipe_info() never be executed.  So AFAICT, this patch
introduces a memory leak, and doesn't actually fix anything...

- Eric