From: Christian Brauner <brauner@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
keescook@chromium.org, axboe@kernel.dk,
christian.koenig@amd.com, dri-devel@lists.freedesktop.org,
io-uring@vger.kernel.org, jack@suse.cz, laura@labbott.name,
linaro-mm-sig@lists.linaro.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
minhquangbui99@gmail.com, sumit.semwal@linaro.org,
syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com,
syzkaller-bugs@googlegroups.com
Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes
Date: Sun, 5 May 2024 12:50:21 +0200 [thread overview]
Message-ID: <20240505-gelehnt-anfahren-8250b487da2c@brauner> (raw)
In-Reply-To: <CAHk-=wirxPSQgRV1u7t4qS1t4ED7w7OeehdUSC-LYZXspqa49w@mail.gmail.com>
On Sat, May 04, 2024 at 08:40:25AM -0700, Linus Torvalds wrote:
> On Sat, 4 May 2024 at 08:32, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Now, during this TOTALLY INNOCENT sock_poll(), in another thread, the
> > file closing completes, eventpoll_release() finishes [..]
>
> Actually, Al is right that ep_item_poll() should be holding the
> ep->mtx, so eventpoll_release() -> eventpoll_release_file_file() ->
> mutex_lock(&ep->mtx) should block and the file doesn't actually get
> released.
So I know you've seen it yourself but for my own peace of mind I've said
that in the other mail and in the other thread already that all callers
of ep_item_poll() do already hold the ep->mtx:
do_epoll_ctl()
-> epoll_mutex_lock(&ep->mtx)
-> ep_insert()
-> ep_item_poll()
do_epoll_ctl()
-> epoll_mutex_lock(&ep->mtx)
-> ep_modify()
-> ep_item_poll()
ep_send_events()
-> mutex_lock(&ep->mtx)
-> ep_item_poll()
/* nested; and all callers of ep_item_poll() already hold ep->mtx */
__ep_eventpoll_poll()
-> mutex_lock_nested(&ep->mtx, wait)
-> ep_item_poll()
So it's simply not possible to end up with a UAF in f_op->poll() because
eventpoll_release_file_file() serializes on ep->mtx as well:
__fput()
-> eventpoll_release()
-> eventpoll_release_file()
{
// @file->f_count is zero _but file is not freed_
// so taking file->f_lock is absolutely fine
spin_lock(&file->f_lock);
// mark as dying
// serialzed on ep->mtx
mutex_lock(&ep->mtx);
__ep_rmove(ep, epi);
...
}
-> mutex_lock(&ep->mtx)
-> f_op->release()
-> kfree(file)
So afaict it's simply not possible to end up with a UAF in f_op->poll().
And I agree with you that for some instances it's valid to take another
reference to a file from f_op->poll() but then they need to use
get_file_active() imho and simply handle the case where f_count is zero.
And we need to document that in Documentation/filesystems/file.rst or
locking.rst.
But if it's simply just dma buf that cares about that long-term
reference then really we should just force them to take the reference
like I suggested but don't penalize everyone else. When I took a glance
at all f_op->poll() implementations I didn't spot one that did take
extra references.
But if you absolutely want to have epoll take the reference before it
calls into f_op->poll() that's fine with me as well. But we might end up
forcing epoll to do a lot of final fput()s which I'm not sure is all
that desirable.
next prev parent reply other threads:[~2024-05-05 10:50 UTC|newest]
Thread overview: 84+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <0000000000002d631f0615918f1e@google.com>
2024-05-03 11:54 ` [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove Bui Quang Minh
2024-05-03 18:26 ` get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove) Kees Cook
2024-05-03 18:49 ` Jens Axboe
2024-05-03 19:22 ` Kees Cook
2024-05-03 19:35 ` Jens Axboe
2024-05-03 19:59 ` Kees Cook
2024-05-03 20:28 ` Kees Cook
2024-05-03 21:11 ` Al Viro
2024-05-03 21:24 ` Linus Torvalds
2024-05-03 21:30 ` Al Viro
2024-05-06 17:46 ` Stefan Metzmacher
2024-05-06 18:17 ` Linus Torvalds
2024-05-08 8:47 ` David Laight
2024-05-03 21:36 ` Al Viro
2024-05-03 21:42 ` Linus Torvalds
2024-05-03 21:53 ` Al Viro
2024-05-06 12:23 ` Daniel Vetter
2024-05-04 9:59 ` Christian Brauner
2024-05-03 21:11 ` [PATCH] epoll: try to be a _bit_ better about file lifetimes Linus Torvalds
2024-05-03 21:24 ` Al Viro
2024-05-03 21:33 ` Linus Torvalds
2024-05-03 21:45 ` Al Viro
2024-05-03 21:52 ` Linus Torvalds
2024-05-03 22:01 ` Al Viro
2024-05-03 22:07 ` Al Viro
2024-05-03 23:16 ` Linus Torvalds
2024-05-03 23:39 ` Al Viro
2024-05-03 23:54 ` Linus Torvalds
2024-05-04 10:44 ` Christian Brauner
2024-05-03 22:46 ` Kees Cook
2024-05-03 23:03 ` Al Viro
2024-05-03 23:23 ` Kees Cook
2024-05-03 23:41 ` Linus Torvalds
2024-05-04 9:19 ` Christian Brauner
2024-05-06 12:37 ` Daniel Vetter
2024-05-04 9:37 ` Christian Brauner
2024-05-04 15:32 ` Linus Torvalds
2024-05-04 15:40 ` Linus Torvalds
2024-05-04 15:53 ` Linus Torvalds
2024-05-05 19:46 ` Al Viro
2024-05-05 20:03 ` Linus Torvalds
2024-05-05 20:30 ` Al Viro
2024-05-05 20:53 ` Linus Torvalds
2024-05-06 12:47 ` Daniel Vetter
2024-05-06 14:46 ` Christian Brauner
2024-05-07 10:58 ` Daniel Vetter
2024-05-06 16:15 ` Christian König
2024-05-05 10:50 ` Christian Brauner [this message]
2024-05-05 16:46 ` Linus Torvalds
2024-05-05 17:55 ` [PATCH v2] epoll: be " Linus Torvalds
2024-05-05 18:04 ` Jens Axboe
2024-05-05 20:01 ` David Laight
2024-05-05 20:16 ` Linus Torvalds
2024-05-05 20:12 ` [PATCH] epoll: try to be a _bit_ " Al Viro
2024-05-06 8:45 ` Christian Brauner
2024-05-06 9:26 ` Christian Brauner
2024-05-06 14:19 ` Christian Brauner
2024-05-07 21:02 ` David Laight
2024-05-04 18:20 ` Linus Torvalds
2024-05-06 14:29 ` [Linaro-mm-sig] " Christian König
2024-05-07 11:02 ` Daniel Vetter
2024-05-07 16:46 ` Linus Torvalds
2024-05-07 17:45 ` Christian König
2024-05-08 7:51 ` Michel Dänzer
2024-05-08 7:59 ` Christian König
2024-05-08 8:23 ` Christian Brauner
2024-05-08 9:10 ` Christian König
2024-05-07 18:04 ` Daniel Vetter
2024-05-07 19:07 ` Linus Torvalds
2024-05-08 5:55 ` Christian König
2024-05-08 8:32 ` Daniel Vetter
2024-05-08 10:16 ` Christian Brauner
2024-05-08 8:05 ` Christian Brauner
2024-05-08 16:19 ` Linus Torvalds
2024-05-08 17:14 ` Linus Torvalds
2024-05-09 11:38 ` Christian Brauner
2024-05-09 15:48 ` Linus Torvalds
2024-05-10 6:33 ` Christian Brauner
2024-05-08 10:08 ` Christian Brauner
2024-05-08 15:45 ` Daniel Vetter
2024-05-10 10:55 ` Christian Brauner
2024-05-11 18:25 ` David Laight
2024-05-05 17:31 ` Jens Axboe
2024-05-04 9:45 ` get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove) Christian Brauner
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