Received-SPF: pass (google.com: domain of linux-kernel+bounces-168265-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Date: Fri, 3 May 2024 22:36:25 +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,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	syzkaller-bugs@googlegroups.com,
	Sumit Semwal <sumit.semwal@linaro.org>,
	Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
	linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org,
	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)
Message-ID: <20240503213625.GA2118490@ZenIV>
References: <0000000000002d631f0615918f1e@google.com>
 <7c41cf3c-2a71-4dbb-8f34-0337890906fc@gmail.com>
 <202405031110.6F47982593@keescook>
 <64b51cc5-9f5b-4160-83f2-6d62175418a2@kernel.dk>
 <202405031207.9D62DA4973@keescook>
 <d6285f19-01aa-49c8-8fef-4b5842136215@kernel.dk>
 <202405031237.B6B8379@keescook>
 <202405031325.B8979870B@keescook>
 <20240503211109.GX2118490@ZenIV>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240503211109.GX2118490@ZenIV>
Sender: Al Viro <viro@ftp.linux.org.uk>

On Fri, May 03, 2024 at 10:11:09PM +0100, Al Viro wrote:
> 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.

.. the last part is no-go - poll_wait() must be able to grab a reference
(well, the callback in it must)