linux-kernel - Re: [PATCH v2 1/3] udmabuf: fix racy memfd sealing check

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Message-ID: <CAEXW_YRUVwWxuoWs1fU8OCsOf+vAWc__csX2Ed0W+yVr0Y49aA@mail.gmail.com>
Date: Tue, 10 Dec 2024 19:04:26 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: Jann Horn <jannh@...gle.com>
Cc: Gerd Hoffmann <kraxel@...hat.com>, Vivek Kasireddy <vivek.kasireddy@...el.com>, 
	Sumit Semwal <sumit.semwal@...aro.org>, Christian König <christian.koenig@....com>, 
	Simona Vetter <simona.vetter@...ll.ch>, John Stultz <jstultz@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, dri-devel@...ts.freedesktop.org, 
	linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org, 
	linux-kernel@...r.kernel.org, Julian Orth <ju.orth@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/3] udmabuf: fix racy memfd sealing check

On Tue, Dec 10, 2024 at 6:12 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Tue, Dec 10, 2024 at 11:51 PM Joel Fernandes <joel@...lfernandes.org> wrote:
> > On Wed, Dec 4, 2024 at 11:27 AM Jann Horn <jannh@...gle.com> wrote:
> > > The current check_memfd_seals() is racy: Since we first do
> > > check_memfd_seals() and then udmabuf_pin_folios() without holding any
> > > relevant lock across both, F_SEAL_WRITE can be set in between.
> > > This is problematic because we can end up holding pins to pages in a
> > > write-sealed memfd.
> > >
> > > Fix it using the inode lock, that's probably the easiest way.
> > > In the future, we might want to consider moving this logic into memfd,
> > > especially if anyone else wants to use memfd_pin_folios().
> >
> > I am curious, why is it not possible to have a reproducer for this
> > issue, is it not reproducible and is theoretical?
>
> Sorry, I think I must have forgotten about this part when I wrote the
> cover letter: The original bug reporter (Julian) linked to a
> reproducer that is linked in the bugzilla bug report, at
> <https://github.com/mahkoh/udmabuf-seal>. I haven't tried running it
> myself though.

Thanks, I appreciate the pointer to the reproducer.

Acked-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel