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Subject: Re: Safety of opening up /dev/dma_heap/* to physically present
 users (udev uaccess tag) ?
From: Nicolas Dufresne <nicolas.dufresne@collabora.com>
To: Maxime Ripard <mripard@redhat.com>, Laurent Pinchart
 <laurent.pinchart@ideasonboard.com>, Bryan O'Donoghue
 <bryan.odonoghue@linaro.org>, Dmitry Baryshkov
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 <benjamin.gaignard@collabora.com>, Brian Starkey <Brian.Starkey@arm.com>, 
 John Stultz <jstultz@google.com>, "T.J. Mercier" <tjmercier@google.com>,
 Christian =?ISO-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>, Lennart
 Poettering <mzxreary@0pointer.de>,  Robert Mader
 <robert.mader@collabora.com>, Sebastien Bacher
 <sebastien.bacher@canonical.com>, Linux Media Mailing List
 <linux-media@vger.kernel.org>, "dri-devel@lists.freedesktop.org"
 <dri-devel@lists.freedesktop.org>,  linaro-mm-sig@lists.linaro.org, Linux
 Kernel Mailing List <linux-kernel@vger.kernel.org>, Milan Zamazal
 <mzamazal@redhat.com>, Andrey Konovalov <andrey.konovalov.ynk@gmail.com>
Date: Mon, 13 May 2024 09:42:00 -0400
In-Reply-To: <20240513-heretic-didactic-newt-1d6daf@penduick>
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Le lundi 13 mai 2024 =C3=A0 10:29 +0200, Maxime Ripard a =C3=A9crit=C2=A0:
> On Wed, May 08, 2024 at 10:36:08AM +0200, Daniel Vetter wrote:
> > On Tue, May 07, 2024 at 04:07:39PM -0400, Nicolas Dufresne wrote:
> > > Hi,
> > >=20
> > > Le mardi 07 mai 2024 =C3=A0 21:36 +0300, Laurent Pinchart a =C3=A9cri=
t=C2=A0:
> > > > Shorter term, we have a problem to solve, and the best option we ha=
ve
> > > > found so far is to rely on dma-buf heaps as a backend for the frame
> > > > buffer allocatro helper in libcamera for the use case described abo=
ve.
> > > > This won't work in 100% of the cases, clearly. It's a stop-gap meas=
ure
> > > > until we can do better.
> > >=20
> > > Considering the security concerned raised on this thread with dmabuf =
heap
> > > allocation not be restricted by quotas, you'd get what you want quick=
ly with
> > > memfd + udmabuf instead (which is accounted already).
> > >=20
> > > It was raised that distro don't enable udmabuf, but as stated there b=
y Hans, in
> > > any cases distro needs to take action to make the softISP works. This
> > > alternative is easy and does not interfere in anyway with your future=
 plan or
> > > the libcamera API. You could even have both dmabuf heap (for Raspbian=
) and the
> > > safer memfd+udmabuf for the distro with security concerns.
> > >=20
> > > And for the long term plan, we can certainly get closer by fixing tha=
t issue
> > > with accounting. This issue also applied to v4l2 io-ops, so it would =
be nice to
> > > find common set of helpers to fix these exporters.
> >=20
> > Yeah if this is just for softisp, then memfd + udmabuf is also what I w=
as
> > about to suggest. Not just as a stopgap, but as the real official thing=
.
> >=20
> > udmabuf does kinda allow you to pin memory, but we can easily fix that =
by
> > adding the right accounting and then either let mlock rlimits or cgroup=
s
> > kernel memory limits enforce good behavior.
>=20
> I think the main drawback with memfd is that it'll be broken for devices
> without an IOMMU, and while you said that it's uncommon for GPUs, it's
> definitely not for codecs and display engines.

In the context of libcamera, the allocation and the alignment done to the v=
ideo
frame is done completely blindly. In that context, there is a lot more then=
 just
the allocation type that can go wrong and will lead to a memory copy. The u=
pside
of memfd, is that the read cache will help speeding up the copies if they a=
re
needed.

Another important point is that this is only used if the application haven'=
t
provided frames. If your embedded application is non-generic, and you have
permissions to access the right heap, the application can solve your specif=
ic
issue. But in the generic Linux space, Linux kernel API are just insufficie=
nt
for the "just work" scenario.

Nicolas