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

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Date: Mon, 6 May 2024 14:05:12 +0200
From: Maxime Ripard <mripard@...hat.com>
To: Hans de Goede <hdegoede@...hat.com>
Cc: Sumit Semwal <sumit.semwal@...aro.org>, 
	Benjamin Gaignard <benjamin.gaignard@...labora.com>, Brian Starkey <Brian.Starkey@....com>, 
	John Stultz <jstultz@...gle.com>, "T.J. Mercier" <tjmercier@...gle.com>, 
	Christian König <christian.koenig@....com>, Lennart Poettering <mzxreary@...inter.de>, 
	Robert Mader <robert.mader@...labora.com>, Sebastien Bacher <sebastien.bacher@...onical.com>, 
	Linux Media Mailing List <linux-media@...r.kernel.org>, 
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, linaro-mm-sig@...ts.linaro.org, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Bryan O'Donoghue <bryan.odonoghue@...aro.org>, 
	Milan Zamazal <mzamazal@...hat.com>, Andrey Konovalov <andrey.konovalov.ynk@...il.com>
Subject: Re: Safety of opening up /dev/dma_heap/* to physically present users
 (udev uaccess tag) ?

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

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