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

[Siddh Raman Pant]

On Wed, 03 Aug 2022 09:42:28 +0530  Eric Biggers <ebiggers at kernel.org> wrote:
> 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.

It really didn't fix the crash. It caused the same crash reported here, which
I was already locally getting:
https://syzkaller.appspot.com/bug?extid=03d7b43290037d1f87ca

(It's same because __post_watch_notification calls post_one_notification, and
this patch seems to stop that crash too, as was verified by Dipanjan here).

While it has been fixed by e64ab2dbd882 ("watch_queue: Fix missing locking in
add_watch_to_object()"), it just shows there can be paths leading to it. (Also,
I posted this patch (v1, v2, v3) before that even landed, so I had no way of
knowing about it).

There is a null check in post_one_notification for the pipe, most probably
because it *expects* the pointer to be NULL'd. Also, there is no reason to have
a dangling pointer stay, it's just a recipe for further bugs.

Thanks,
Siddh