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

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