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

[Dmitry Baryshkov]

On Mon, May 06, 2024 at 01:49:17PM +0200, 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

Is there any reason for allocating DMA buffers for libcamera through
/dev/dma_heap/ rather than allocating them via corresponding media
device and then giving them away to DRM / VPU / etc?

At least this should solve the permission usecase: if the app can open
camera device, it can allocate a buffer.

> 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.
> 
> 

-- 
With best wishes
Dmitry