linux-kernel - Re: [PATCH] epoll: try to be a _bit

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Message-ID: <202405031529.2CD1BFED37@keescook>
Date: Fri, 3 May 2024 15:46:25 -0700
From: Kees Cook <keescook@...omium.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Al Viro <viro@...iv.linux.org.uk>, axboe@...nel.dk, brauner@...nel.org,
	christian.koenig@....com, dri-devel@...ts.freedesktop.org,
	io-uring@...r.kernel.org, jack@...e.cz, laura@...bott.name,
	linaro-mm-sig@...ts.linaro.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	minhquangbui99@...il.com, sumit.semwal@...aro.org,
	syzbot+045b454ab35fd82a35fb@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

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...

-- 
Kees Cook