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

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Date: Mon, 6 May 2024 14:23:48 +0200
From: Daniel Vetter <daniel@...ll.ch>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	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, May 03, 2024 at 10:53:03PM +0100, Al Viro wrote:
> On Fri, May 03, 2024 at 02:42:22PM -0700, Linus Torvalds wrote:
> > On Fri, 3 May 2024 at 14:36, Al Viro <viro@...iv.linux.org.uk> wrote:
> > >
> > > ... the last part is no-go - poll_wait() must be able to grab a reference
> > > (well, the callback in it must)
> > 
> > Yeah. I really think that *poll* itself is doing everything right. It
> > knows that it's called with a file pointer with a reference, and it
> > adds its own references as needed.
> 
> Not really.  Note that select's __pollwait() does *NOT* leave a reference
> at the mercy of driver - it's stuck into poll_table_entry->filp and
> the poll_freewait() knows how to take those out.
> 
> 
> dmabuf does something very different - it grabs the damn thing into
> its private data structures and for all we know it could keep it for
> a few hours, until some even materializes.

dma_fence must complete in reasonable amount of time, where "reasonable"
is roughly in line with other i/o (including the option that there's
timeouts if the hw's gone busted).

So definitely not hours (aside from driver bugs when things go really
wrong ofc), but more like a few seconds in a worst case scenario.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch