Safety of opening up /dev/dma_heap/* to physically present users (udev uaccess tag) ?

[Maxime Ripard]

Hi,

On Mon, May 06, 2024 at 03:38:24PM GMT, Daniel Vetter wrote:
> On Mon, May 06, 2024 at 02:05:12PM +0200, Maxime Ripard wrote:
> > Hi,
> > 
> > On Mon, May 06, 2024 at 01:49:17PM GMT, Hans de Goede wrote:
> > > Hi dma-buf maintainers, et.al.,
> > > 
> > > Various people have been working on making complex/MIPI cameras work OOTB
> > > with mainline Linux kernels and an opensource userspace stack.
> > > 
> > > The generic solution adds a software ISP (for Debayering and 3A) to
> > > libcamera. Libcamera's API guarantees that buffers handed to applications
> > > using it are dma-bufs so that these can be passed to e.g. a video encoder.
> > > 
> > > In order to meet this API guarantee the libcamera software ISP allocates
> > > dma-bufs from userspace through one of the /dev/dma_heap/* heaps. For
> > > the Fedora COPR repo for the PoC of this:
> > > https://hansdegoede.dreamwidth.org/28153.html
> > 
> > For the record, we're also considering using them for ARM KMS devices,
> > so it would be better if the solution wasn't only considering v4l2
> > devices.
> > 
> > > I have added a simple udev rule to give physically present users access
> > > to the dma_heap-s:
> > > 
> > > KERNEL=="system", SUBSYSTEM=="dma_heap", TAG+="uaccess"
> > > 
> > > (and on Rasperry Pi devices any users in the video group get access)
> > > 
> > > This was just a quick fix for the PoC. Now that we are ready to move out
> > > of the PoC phase and start actually integrating this into distributions
> > > the question becomes if this is an acceptable solution; or if we need some
> > > other way to deal with this ?
> > > 
> > > Specifically the question is if this will have any negative security
> > > implications? I can certainly see this being used to do some sort of
> > > denial of service attack on the system (1). This is especially true for
> > > the cma heap which generally speaking is a limited resource.
> > 
> > There's plenty of other ways to exhaust CMA, like allocating too much
> > KMS or v4l2 buffers. I'm not sure we should consider dma-heaps
> > differently than those if it's part of our threat model.
> 
> So generally for an arm soc where your display needs cma, your render node
> doesn't. And user applications only have access to the later, while only
> the compositor gets a kms fd through logind. At least in drm aside from
> vc4 there's really no render driver that just gives you access to cma and
> allows you to exhaust that, you need to be a compositor with drm master
> access to the display.
> 
> Which means we're mostly protected against bad applications, and that's
> not a threat the "user physically sits in front of the machine accounts
> for", and which giving cma access to everyone would open up. And with
> flathub/snaps/... this is very much an issue.
> 
> So you need more, either:
> 
> - cgroups limits on dma-buf and dma-buf heaps. This has been bikeshedded
>   for years and is just not really moving.

For reference, are you talking about:

https://lore.kernel.org/r/20220502231944.3891435-1-tjmercier@google.com

Or has there been a new version of that recently?

Maxime
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