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 22:53:03 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
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, 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.