From: "Christian König" <christian.koenig@amd.com>
To: David Laight <David.Laight@ACULAB.COM>,
"T.J. Mercier" <tjmercier@google.com>,
Charan Teja Kalla <quic_charante@quicinc.com>,
zhiguojiang <justinjiang@vivo.com>,
Sumit Semwal <sumit.semwal@linaro.org>,
"linux-media@vger.kernel.org" <linux-media@vger.kernel.org>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
"linaro-mm-sig@lists.linaro.org" <linaro-mm-sig@lists.linaro.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"opensource.kernel@vivo.com" <opensource.kernel@vivo.com>
Subject: Re: [PATCH] dmabuf: fix dmabuf file poll uaf issue
Date: Wed, 8 May 2024 14:21:58 +0200 [thread overview]
Message-ID: <facb101b-3535-421d-bffb-c4ea675c9107@amd.com> (raw)
In-Reply-To: <c0fe95949d4f41449f17add8300270b9@AcuMS.aculab.com>
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Am 08.05.24 um 13:51 schrieb David Laight:
> From: Christian König
>> Sent: 07 May 2024 15:05
> ...
>> I actually have been telling people to (ab)use the epoll behavior to
>> check if two file descriptors point to the same underlying file when
>> KCMP isn't available.
> In what way?
Something like this:
fd_e = epoll_create1(EPOLL_CLOEXEC);
tmp = dup(fd_A)
epoll_ctl(fd_e, EPOLL_CTL_ADD, tmp, ....);
dup2(fd_B, tmp);
/* If this return -EEXISTS then the fd_A and fd_B are pointing to the
same struct file */
epoll_ctl(fd_e, EPOLL_CTL_ADD, tmp, ....);
close (tmp);
close (fd_e
> You can add both fd to the same epoll fd.
> Relying on the implicit EPOLL_CTL_DEL not happening until both fd are
> closed is a recipe for disaster.
> (And I can't see an obvious way of testing it.)
>
> Q6/A6 on epoll(7) should always have had a caveat that it is an
> 'implementation detail' and shouldn't be relied on.
> (it is written as a 'beware of' ...)
>
> The other point is that there are two ways to get multiple fd that
> reference the same underlying file.
> dup() fork() etc share the file offset, but open("/dev/fd/n") adds
> a reference count later and has a separate file offset.
No it doesn't.
Accessing /dev/fd/n or /proc/*/fd/n ideally accesses the same inode, but
gives you a new struct file.
dup(), fork() etc.. make you actually reference the same struct file
inside the kernel.
That turned out to be a rather important distinction when working with
device drivers and DMA-buf.
Regards,
Christian.
>
> I don't know which structure epoll is using, but I suspect it is
> the former.
> So it may not tell you what you want to know.
>
> David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
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prev parent reply other threads:[~2024-05-08 12:22 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-27 2:29 [PATCH] dmabuf: fix dmabuf file poll uaf issue Zhiguo Jiang
2024-03-29 23:36 ` T.J. Mercier
2024-04-01 6:52 ` zhiguojiang
2024-04-01 12:22 ` Christian König
2024-04-02 6:49 ` zhiguojiang
2024-04-02 8:07 ` Christian König
2024-04-02 18:22 ` T.J. Mercier
2024-04-12 6:19 ` zhiguojiang
2024-04-12 6:39 ` Christian König
2024-04-15 10:35 ` zhiguojiang
2024-04-15 11:57 ` Christian König
2024-04-18 1:33 ` zhiguojiang
2024-04-18 6:46 ` Christian König
2024-05-03 13:40 ` Charan Teja Kalla
2024-05-03 23:13 ` T.J. Mercier
2024-05-05 16:20 ` Charan Teja Kalla
2024-05-06 9:30 ` Charan Teja Kalla
2024-05-06 19:04 ` T.J. Mercier
2024-05-07 10:10 ` Christian König
2024-05-07 13:39 ` Daniel Vetter
2024-05-07 14:04 ` Christian König
2024-05-07 18:00 ` T.J. Mercier
2024-05-07 20:19 ` Rob Clark
2024-05-08 11:51 ` David Laight
2024-05-08 12:21 ` Christian König [this message]
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