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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>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>, Bryan O'Donoghue
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Date: Mon, 13 May 2024 11:06:24 -0400
In-Reply-To: <20240513-auspicious-toucanet-from-heaven-f313af@penduick>
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Le lundi 13 mai 2024 =C3=A0 15:51 +0200, Maxime Ripard a =C3=A9crit=C2=A0:
> On Mon, May 13, 2024 at 09:42:00AM -0400, Nicolas Dufresne wrote:
> > 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=
=A9crit=C2=A0:
> > > > > > Shorter term, we have a problem to solve, and the best option w=
e have
> > > > > > found so far is to rely on dma-buf heaps as a backend for the f=
rame
> > > > > > buffer allocatro helper in libcamera for the use case described=
 above.
> > > > > > This won't work in 100% of the cases, clearly. It's a stop-gap =
measure
> > > > > > until we can do better.
> > > > >=20
> > > > > Considering the security concerned raised on this thread with dma=
buf heap
> > > > > allocation not be restricted by quotas, you'd get what you want q=
uickly with
> > > > > memfd + udmabuf instead (which is accounted already).
> > > > >=20
> > > > > It was raised that distro don't enable udmabuf, but as stated the=
re by 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 fu=
ture plan or
> > > > > the libcamera API. You could even have both dmabuf heap (for Rasp=
bian) 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=
 that issue
> > > > > with accounting. This issue also applied to v4l2 io-ops, so it wo=
uld 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 was
> > > > about to suggest. Not just as a stopgap, but as the real official t=
hing.
> > > >=20
> > > > udmabuf does kinda allow you to pin memory, but we can easily fix t=
hat by
> > > > adding the right accounting and then either let mlock rlimits or cg=
roups
> > > > kernel memory limits enforce good behavior.
> > >=20
> > > I think the main drawback with memfd is that it'll be broken for devi=
ces
> > > without an IOMMU, and while you said that it's uncommon for GPUs, it'=
s
> > > definitely not for codecs and display engines.
> >=20
> > In the context of libcamera, the allocation and the alignment done to t=
he video
> > 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. T=
he upside
> > of memfd, is that the read cache will help speeding up the copies if th=
ey are
> > needed.
>=20
> dma-heaps provide cacheable buffers too...

Yes, and why we have cache hints in V4L2 now. There is no clue that softISP=
 code
can read to make the right call. The required cache management in undefined
until all the importer are known. I also don't think heaps currently care t=
o
adapt the dmabuf sync behaviour based on the different importers, or the
addition of a new importer. On top of which, there is insufficient informat=
ion
on the device to really deduce what is needed.

>=20
> > Another important point is that this is only used if the application ha=
ven't
> > provided frames. If your embedded application is non-generic, and you h=
ave
> > permissions to access the right heap, the application can solve your sp=
ecific
> > issue. But in the generic Linux space, Linux kernel API are just insuff=
icient
> > for the "just work" scenario.
>=20
> ... but they also provide semantics around the memory buffers that no
> other allocation API do. There's at least the mediatek secure playback
> series and another one that I've started to work on to allocate ECC
> protected or unprotected buffers that are just the right use case for
> the heaps, and the target frameworks aren't.

Let's agree we are both off topic now. The libcamera softISP is currently p=
urely
software, and cannot write to any form of protected memory. As for ECC, I w=
ould
hope this usage will be coded in the application and that this application =
has
been authorized to access the appropriate heaps.

And finally, none of this fixes the issue that the heap allocation are not =
being
accounted properly and allow of an easy memory DoS. So uaccess should be gr=
anted
with care, meaning that defaulting a "desktop" library to that, means it wi=
ll
most of the time not work at all.

Nicolas