[RFC PATCH] drm:- Add a modifier to denote 'protected' framebuffer

[Daniel Vetter]

On Thu, Sep 19, 2019 at 4:03 PM Ayan Halder <Ayan.Halder at arm.com> wrote:
>
> On Wed, Sep 18, 2019 at 10:30:12PM +0100, Daniel Stone wrote:
>
> Hi All,
> Thanks for your suggestions.
>
> > Hi Liviu,
> >
> > On Wed, 18 Sep 2019 at 13:04, Liviu Dudau <Liviu.Dudau at arm.com> wrote:
> > > On Wed, Sep 18, 2019 at 09:49:40AM +0100, Daniel Stone wrote:
> > > > I totally agree. Framebuffers aren't about the underlying memory they
> > > > point to, but about how to _interpret_ that memory: it decorates a
> > > > pointer with width, height, stride, format, etc, to allow you to make
> > > > sense of that memory. I see content protection as being the same as
> > > > physical contiguity, which is a property of the underlying memory
> > > > itself. Regardless of the width, height, or format, you just cannot
> > > > access that memory unless it's under the appropriate ('secure enough')
> > > > conditions.
> > > >
> > > > So I think a dmabuf attribute would be most appropriate, since that's
> > > > where you have to do all your MMU handling, and everything else you
> > > > need to do to allow access to that buffer, anyway.
> > >
> > > Isn't it how AMD currently implements protected buffers as well?
> >
> > No idea to be honest, I haven't seen anything upstream.
> >
> > > > There's a lot of complexity beyond just 'it's protected'; for
> > > > instance, some CP providers mandate that their content can only be
> > > > streamed over HDCP 2.2 Type-1 when going through an external
> > > > connection. One way you could do that is to use a secure world
> > > > (external controller like Intel's ME, or CPU-internal enclave like SGX
> > > > or TEE) to examine the display pipe configuration, and only then allow
> > > > access to the buffers and/or keys. Stuff like that is always going to
> > > > be out in the realm of vendor & product-policy-specific add-ons, but
> > > > it should be possible to agree on at least the basic mechanics and
> > > > expectations of a secure path without things like that.
> > >
> > > I also expect that going through the secure world will be pretty much transparent for
> > > the kernel driver, as the most likely hardware implementations would enable
> > > additional signaling that will get trapped and handled by the secure OS. I'm not
> > > trying to simplify things, just taking the stance that it is userspace that is
> > > coordinating all this, we're trying only to find a common ground on how to handle
> > > this in the kernel.
> >
> > Yeah, makes sense.
> >
> > As a strawman, how about a new flag to drmPrimeHandleToFD() which sets
> > the 'protected' flag on the resulting dmabuf?
>
> To be honest, during our internal discussion james.qian.wang at arm.com had a
> similar suggestion of adding a new flag to dma_buf but I decided
> against it.
>
> As per my understanding, adding custom dma buf flags (like
> AMDGPU_GEM_CREATE_XXX, etc) is possible if we(Arm) had our own allocator. We
> rely on the dumb allocator and ion allocator for framebuffer creation.
>
> I was looking at an allocator independent way of userspace
> communicating to the drm framework that the framebuffer is protected.
>
> Thus, it looked to me that framebuffer modifier is the best (or the least
> corrupt) way of going forth.
>
> We use ion and dumb allocator for framebuffer object creation. The way
> I see it is as follows :-
>
> 1. For ion allocator :-
> Userspace can specify that it wants the buffer from a secure heap or any other
> special region of heap. The ion driver will either fault into the secure os to
> create the buffers or it will do some other magic. Honestly, I have still not
> figured that out. But it will be agnostic to the drm core.

Allocating buffers from a special heap is what I expected the
interface to be. The issue is that if we specify the secure mode any
time later on, then it could be changed. E.g. with Daniel Stone's idea
of a handle2fd flag, you could export the buffer twice, once secure,
once non-secure. That sounds like a silly thing to me, and better to
prevent that - or is this actually possible/wanted, i.e. do we want to
change the secure mode for a buffer later on?

> The userspace gets a handle to the buffer and then it calls addFB2 with
> DRM_FORMAT_MOD_ARM_PROTECTED, so that the driver can configure the
> dpu's protection bits (to access the memory with special signals).

If we allocate a secure buffer there's no need for flags anymore I
think - it would be a property of the underlying buffer (like a
contiguous buffer). All we need are two things:
- make sure secure buffers can only be imported by secure-buffer aware drivers
- some way for such drivers to figure out whether they deal with a
secure buffer or not.

There's no need for any flags anywhere else with the ion/secure
dma-buf heap solution. E.g. for contig buffer we also dont pass around
a DRM_FORMAT_MOD_PHYSICALLY_CONTIG for addfb2.

> 2. For dumb allocator :-
> I am curious to know if we can add 'IS_PROTECTED' flag to
> drm_mode_create_dumb.flags. This can/might be used to set dma_buf
> flags. Let me know if this is an incorrect/forbidden path.

dumb is dumb, this definitely feels like the wrong interface to me.

> In a nutshell, my objective is to figure out if the userspace is able
> to communicate to the drm core about the 'protection' status of the
> buffer without introducing Arm specific buffer allocator.

Why does userspace need to communicate this again? What's the issue
with using an ARM specific allocator for this?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch