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

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Date: Mon, 6 May 2024 13:49:17 +0200
From: Hans de Goede <hdegoede@...hat.com>
To: 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>
Cc: 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>, Maxime Ripard <mripard@...hat.com>,
 Andrey Konovalov <andrey.konovalov.ynk@...il.com>
Subject: Safety of opening up /dev/dma_heap/* to physically present users
 (udev uaccess tag) ?

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

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.

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. Any accidental DOS would be a userspace stack bug.

Do you foresee any other negative security implications from allowing
physically present non root users to create (u)dma-bufs ?

Regards,

Hans

1) There are some limits in drivers/dma-buf/udmabuf.c and distributions
could narrow these.