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

[Hans de Goede <hdegoede@redhat.com>]

Hi Maxime,

On 5/6/24 2:05 PM, 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.

Ack.

>> 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?

The uaccess tag solution does not work for headless devices, but it
also should not hurt any headless scenarios.

Headless devices could use something like the video group solution
(dma_heap group?) which Raspberry Pi is using and them make sure that
any services which need access run as a user in that group.

This can co-exist with uaccess tags since those use ACLs not classic Unix
permissions.

Regards,

Hans