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

Maxime Ripard
2024-05-06T12:05:18Z

[-- Attachment #1: Type: text/plain, Size: 2156 bytes --]

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.

> But devices tagged for uaccess are only opened up to users who are
> physcially present behind the machine and those can just hit
> the powerbutton, so I don't believe that any *on purpose* DOS is part of
> the thread model.

How would that work for headless devices?

Maxime

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 273 bytes --]