linux-kernel - Re: [PATCH] epoll: try to be a _bit

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Date: Fri, 3 May 2024 22:45:31 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: keescook@...omium.org, axboe@...nel.dk, brauner@...nel.org,
	christian.koenig@....com, dri-devel@...ts.freedesktop.org,
	io-uring@...r.kernel.org, jack@...e.cz, laura@...bott.name,
	linaro-mm-sig@...ts.linaro.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	minhquangbui99@...il.com, sumit.semwal@...aro.org,
	syzbot+045b454ab35fd82a35fb@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

On Fri, May 03, 2024 at 02:33:37PM -0700, Linus Torvalds wrote:

> Look at the hack in __ep_remove(): if it is concurrent with
> eventpoll_release_file(), it will hit this code
> 
>         spin_lock(&file->f_lock);
>         if (epi->dying && !force) {
>                 spin_unlock(&file->f_lock);
>                 return false;
>         }
> 
> and not free the epi.

What does that have to do with ep_item_poll()?

eventpoll_release_file() itself calls __ep_remove().  Have that
happen while ep_item_poll() is running in another thread and
you've got a problem.

AFAICS, exclusion is on ep->mtx.  Callers of ep_item_poll() are
* __ep_eventpoll_poll() - grabs ->mtx
* ep_insert() - called under ->mtx
* ep_modify() - calls are under ->mtx
* ep_send_events() - grabs ->mtx

and eventpoll_release_file() grabs ->mtx around __ep_remove().

How do you get through eventpoll_release_file() while someone
has entered ep_item_poll()?