Received-SPF: pass (google.com: domain of linux-kernel+bounces-168254-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Date: Fri, 3 May 2024 22:30:48 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>, 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>
Subject: Re: get_file() unsafe under epoll (was Re: [syzbot] [fs?]
 [io-uring?] general protection fault in __ep_remove)
Message-ID: <20240503213048.GZ2118490@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>
 <CAHk-=wj0de-P2Q=Gz2uyrWBHagT25arLbN0Lyg=U6fT7psKnQA@mail.gmail.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=wj0de-P2Q=Gz2uyrWBHagT25arLbN0Lyg=U6fT7psKnQA@mail.gmail.com>
Sender: Al Viro <viro@ftp.linux.org.uk>

On Fri, May 03, 2024 at 02:24:45PM -0700, Linus Torvalds wrote:
> 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().

eventpoll_release_file() calling __ep_remove() while ep_item_poll()
is something we need to avoid anyway - having epi freed under
ep_item_poll() would be a problem regardless of struct file
lifetime issues.