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

Nicolas Dufresne
2024-05-13T13:42:08Z

Le lundi 13 mai 2024 à 10:29 +0200, Maxime Ripard a écrit :
> On Wed, May 08, 2024 at 10:36:08AM +0200, Daniel Vetter wrote:
> > On Tue, May 07, 2024 at 04:07:39PM -0400, Nicolas Dufresne wrote:
> > > Hi,
> > >
> > > Le mardi 07 mai 2024 à 21:36 +0300, Laurent Pinchart a écrit :
> > > > Shorter term, we have a problem to solve, and the best option we have
> > > > found so far is to rely on dma-buf heaps as a backend for the frame
> > > > buffer allocatro helper in libcamera for the use case described above.
> > > > This won't work in 100% of the cases, clearly. It's a stop-gap measure
> > > > until we can do better.
> > >
> > > Considering the security concerned raised on this thread with dmabuf heap
> > > allocation not be restricted by quotas, you'd get what you want quickly with
> > > memfd + udmabuf instead (which is accounted already).
> > >
> > > It was raised that distro don't enable udmabuf, but as stated there by Hans, in
> > > any cases distro needs to take action to make the softISP works. This
> > > alternative is easy and does not interfere in anyway with your future plan or
> > > the libcamera API. You could even have both dmabuf heap (for Raspbian) and the
> > > safer memfd+udmabuf for the distro with security concerns.
> > >
> > > And for the long term plan, we can certainly get closer by fixing that issue
> > > with accounting. This issue also applied to v4l2 io-ops, so it would be nice to
> > > find common set of helpers to fix these exporters.
> >
> > Yeah if this is just for softisp, then memfd + udmabuf is also what I was
> > about to suggest. Not just as a stopgap, but as the real official thing.
> >
> > udmabuf does kinda allow you to pin memory, but we can easily fix that by
> > adding the right accounting and then either let mlock rlimits or cgroups
> > kernel memory limits enforce good behavior.
>
> I think the main drawback with memfd is that it'll be broken for devices
> without an IOMMU, and while you said that it's uncommon for GPUs, it's
> definitely not for codecs and display engines.

In the context of libcamera, the allocation and the alignment done to the video
frame is done completely blindly. In that context, there is a lot more then just
the allocation type that can go wrong and will lead to a memory copy. The upside
of memfd, is that the read cache will help speeding up the copies if they are
needed.

Another important point is that this is only used if the application haven't
provided frames. If your embedded application is non-generic, and you have
permissions to access the right heap, the application can solve your specific
issue. But in the generic Linux space, Linux kernel API are just insufficient
for the "just work" scenario.

Nicolas