linux-kernel - Re: get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep

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Date: Fri, 3 May 2024 14:24:45 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Kees Cook <keescook@...omium.org>, Jens Axboe <axboe@...nel.dk>, 
	Bui Quang Minh <minhquangbui99@...il.com>, Christian Brauner <brauner@...nel.org>, 
	syzbot <syzbot+045b454ab35fd82a35fb@...kaller.appspotmail.com>, 
	io-uring@...r.kernel.org, jack@...e.cz, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com, 
	Sumit Semwal <sumit.semwal@...aro.org>, Christian König <christian.koenig@....com>, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, Laura Abbott <laura@...bott.name>
Subject: Re: get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?]
 general protection fault in __ep_remove)

On Fri, 3 May 2024 at 14:11, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> What we need is
>         * promise that ep_item_poll() won't happen after eventpoll_release_file().
> AFAICS, we do have that.
>         * ->poll() not playing silly buggers.

No. That is not enough at all.

Because even with perfectly normal "->poll()", and even with the
ep_item_poll() happening *before* eventpoll_release_file(), you have
this trivial race:

  ep_item_poll()
     ->poll()

and *between* those two operations, another CPU does "close()", and
that causes eventpoll_release_file() to be called, and now f_count
goes down to zero while ->poll() is running.

So you do need to increment the file count around the ->poll() call, I feel.

Or, alternatively, you'd need to serialize with
eventpoll_release_file(), but that would need to be some sleeping lock
held over the ->poll() call.

> As it is, dma_buf ->poll() is very suspicious regardless of that
> mess - it can grab reference to file for unspecified interval.

I think that's actually much preferable to what epoll does, which is
to keep using files without having reference counts to them (and then
relying on magically not racing with eventpoll_release_file().

                Linus