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Date: Fri, 3 May 2024 22:11:09 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Kees Cook <keescook@chromium.org>
Cc: Jens Axboe <axboe@kernel.dk>, Bui Quang Minh <minhquangbui99@gmail.com>,
	Christian Brauner <brauner@kernel.org>,
	syzbot <syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com>,
	io-uring@vger.kernel.org, jack@suse.cz,
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	linaro-mm-sig@lists.linaro.org, Laura Abbott <laura@labbott.name>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: get_file() unsafe under epoll (was Re: [syzbot] [fs?]
 [io-uring?] general protection fault in __ep_remove)
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On Fri, May 03, 2024 at 01:28:37PM -0700, Kees Cook wrote:
> 
> Is this the right approach? It still feels to me like get_file() needs
> to happen much earlier...

I don't believe it needs to happen at all.  The problem is not that
->release() can be called during ->poll() - it can't and it doesn't.
It's that this instance of ->poll() is trying to extend the lifetime
of that struct file, when it might very well be past the point of no
return.

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.

As it is, dma_buf ->poll() is very suspicious regardless of that
mess - it can grab reference to file for unspecified interval.
Have that happen shortly before reboot and you are asking for failing
umount.

->poll() must be refcount-neutral wrt file passed to it.  I'm seriously
tempted to make ->poll() take const struct file * and see if there's
anything else that would fall out.