From: Kees Cook <keescook@chromium.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
axboe@kernel.dk, brauner@kernel.org, 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: Fri, 3 May 2024 16:23:33 -0700 [thread overview]
Message-ID: <202405031616.793DF7EEE@keescook> (raw)
In-Reply-To: <20240503230318.GF2118490@ZenIV>
On Sat, May 04, 2024 at 12:03:18AM +0100, Al Viro wrote:
> On Fri, May 03, 2024 at 03:46:25PM -0700, Kees Cook wrote:
> > On Fri, May 03, 2024 at 02:52:38PM -0700, Linus Torvalds wrote:
> > > That means that the file will be released - and it means that you have
> > > violated all the refcounting rules for poll().
> >
> > I feel like I've been looking at this too long. I think I see another
> > problem here, but with dmabuf even when epoll is fixed:
> >
> > dma_buf_poll()
> > get_file(dmabuf->file) /* f_count + 1 */
> > dma_buf_poll_add_cb()
> > dma_resv_for_each_fence ...
> > dma_fence_add_callback(fence, ..., dma_buf_poll_cb)
> >
> > dma_buf_poll_cb()
> > ...
> > fput(dmabuf->file); /* f_count - 1 ... for each fence */
> >
> > Isn't it possible to call dma_buf_poll_cb() (and therefore fput())
> > multiple times if there is more than 1 fence? Perhaps I've missed a
> > place where a single struct dma_resv will only ever signal 1 fence? But
> > looking through dma_fence_signal_timestamp_locked(), I don't see
> > anything about resv nor somehow looking into other fence cb_list
> > contents...
>
> At a guess,
> r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb);
> if (!r)
> return true;
>
> prevents that - it returns 0 on success and -E... on error;
> insertion into the list happens only when it's returning 0,
> so...
Yes; thank you. I *have* been looking at it all too long. :)
The last related thing is the drivers/gpu/drm/vmwgfx/ttm_object.c case:
/**
* get_dma_buf_unless_doomed - get a dma_buf reference if possible.
*
* @dmabuf: Non-refcounted pointer to a struct dma-buf.
*
* Obtain a file reference from a lookup structure that doesn't refcount
* the file, but synchronizes with its release method to make sure it
* has
* not been freed yet. See for example kref_get_unless_zero
* documentation.
* Returns true if refcounting succeeds, false otherwise.
*
* Nobody really wants this as a public API yet, so let it mature here
* for some time...
*/
static bool __must_check get_dma_buf_unless_doomed(struct dma_buf *dmabuf)
{
return atomic_long_inc_not_zero(&dmabuf->file->f_count) != 0L;
}
If we end up adding epi_fget(), we'll have 2 cases of using
"atomic_long_inc_not_zero" for f_count. Do we need some kind of blessed
helper to live in file.h or something, with appropriate comments?
--
Kees Cook
next prev parent reply other threads:[~2024-05-03 23:23 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 [this message]
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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