From: Christian Brauner <[email protected]>
To: Kees Cook <[email protected]>
Cc: "Jens Axboe" <[email protected]>,
"Bui Quang Minh" <[email protected]>,
"Al Viro" <[email protected]>,
syzbot <[email protected]>,
[email protected], [email protected],
[email protected], [email protected],
[email protected],
"Sumit Semwal" <[email protected]>,
"Christian König" <[email protected]>,
[email protected], [email protected],
[email protected],
"Laura Abbott" <[email protected]>
Subject: Re: get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove)
Date: Sat, 4 May 2024 11:59:09 +0200 [thread overview]
Message-ID: <20240504-irrsinn-sinnlich-83cf0890c7dc@brauner> (raw)
In-Reply-To: <202405031237.B6B8379@keescook>
On Fri, May 03, 2024 at 12:59:52PM -0700, Kees Cook wrote:
> On Fri, May 03, 2024 at 01:35:09PM -0600, Jens Axboe wrote:
> > On 5/3/24 1:22 PM, Kees Cook wrote:
> > > On Fri, May 03, 2024 at 12:49:11PM -0600, Jens Axboe wrote:
> > >> On 5/3/24 12:26 PM, Kees Cook wrote:
> > >>> Thanks for doing this analysis! I suspect at least a start of a fix
> > >>> would be this:
> > >>>
> > >>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > >>> index 8fe5aa67b167..15e8f74ee0f2 100644
> > >>> --- a/drivers/dma-buf/dma-buf.c
> > >>> +++ b/drivers/dma-buf/dma-buf.c
> > >>> @@ -267,9 +267,8 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
> > >>>
> > >>> if (events & EPOLLOUT) {
> > >>> /* Paired with fput in dma_buf_poll_cb */
> > >>> - get_file(dmabuf->file);
> > >>> -
> > >>> - if (!dma_buf_poll_add_cb(resv, true, dcb))
> > >>> + if (!atomic_long_inc_not_zero(&dmabuf->file) &&
> > >>> + !dma_buf_poll_add_cb(resv, true, dcb))
> > >>> /* No callback queued, wake up any other waiters */
> > >>
> > >> Don't think this is sane at all. I'm assuming you meant:
> > >>
> > >> atomic_long_inc_not_zero(&dmabuf->file->f_count);
> > >
> > > Oops, yes, sorry. I was typed from memory instead of copy/paste.
> >
> > Figured :-)
> >
> > >> but won't fly as you're not under RCU in the first place. And what
> > >> protects it from being long gone before you attempt this anyway? This is
> > >> sane way to attempt to fix it, it's completely opposite of what sane ref
> > >> handling should look like.
> > >>
> > >> Not sure what the best fix is here, seems like dma-buf should hold an
> > >> actual reference to the file upfront rather than just stash a pointer
> > >> and then later _hope_ that it can just grab a reference. That seems
> > >> pretty horrible, and the real source of the issue.
> > >
> > > AFAICT, epoll just doesn't hold any references at all. It depends,
> > > I think, on eventpoll_release() (really eventpoll_release_file())
> > > synchronizing with epoll_wait() (but I don't see how this happens, and
> > > the race seems to be against ep_item_poll() ...?)
> > >
> > > I'm really confused about how eventpoll manages the lifetime of polled
> > > fds.
> >
> > epoll doesn't hold any references, and it's got some ugly callback to
> > deal with that. It's not ideal, nor pretty, but that's how it currently
> > works. See eventpoll_release() and how it's called. This means that
> > epoll itself is supposedly safe from the file going away, even though it
> > doesn't hold a reference to it.
>
> Right -- what remains unclear to me is how struct file lifetime is
> expected to work in the struct file_operations::poll callbacks. Because
> using get_file() there looks clearly unsafe...
If you're in ->poll() you're holding the epoll mutex and
eventpoll_release_file() needs to acquire ep->mtx as well. So if you're
in ->poll() then you know that eventpoll_release_file() can't progress
and therefore eventpoll_release() can't make progress. So
f_op->release() won't be able to be called as it happens after
eventpoll_release() in __fput(). But f_count being able to go to zero is
expected.
next prev parent reply other threads:[~2024-05-04 9:59 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-08 8:26 [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove syzbot
2024-04-15 14:31 ` Jens Axboe
2024-04-15 14:57 ` Pavel Begunkov
2024-05-03 11:54 ` 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 [this message]
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
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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