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

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Message-ID: <CAHk-=wjpsTEkHgo1uev3xGJ2bQXYShaRf3GPEqDWNgUuKx0JFw@mail.gmail.com>
Date: Fri, 3 May 2024 14:33:37 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Al Viro <viro@...iv.linux.org.uk>
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, 3 May 2024 at 14:24, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Can we get to ep_item_poll(epi, ...) after eventpoll_release_file()
> got past __ep_remove()?  Because if we can, we have a worse problem -
> epi freed under us.

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.

But as far as I can tell, almost nothing else cares about the f_lock
and dying logic.

And in fact, I don't think doing

        spin_lock(&file->f_lock);

is even valid in the places that look up file through "epi->ffd.file",
because the lock itself is inside the thing that you can't trust until
you've taken the lock...

So I agree with Kees about the use of "atomic_dec_not_zero()" kind of
logic - but it also needs to be in an RCU-readlocked region, I think.

I wish epoll() just took the damn file ref itself. But since it relies
on the file refcount to release the data structure, that obviously
can't work.

                Linus