Re: [PATCH] KVM: arm64: Validate the FF-A memory access descriptor placement

[Sebastian Ene <sebastianene@google.com>]

On Wed, Apr 22, 2026 at 01:24:02PM +0100, Marc Zyngier wrote:
> On Wed, 22 Apr 2026 11:25:40 +0100,
> Sebastian Ene <sebastianene@google.com> wrote:
> > 
> > Prevent the pKVM hypervisor from making assumptions that the
> > endpoint memory access descriptor (EMAD) comes right after the
> > FF-A memory region header and enforce a strict placement for it
> > when validating an FF-A memory lend/share transaction.

Hello Marc,

> 
> As I read this, you want to remove a bad assumption...
> 
> > 
> > Prior to FF-A version 1.1 the header of the memory region
> > didn't contain an offset to the endpoint memory access descriptor.
> > The layout of a memory transaction looks like this:
> > 
> >   Field name				| Offset
> > 					 -- 0
> > [ Header (ffa_mem_region)               |__ ep_mem_offset
> >   EMAD 1 (ffa_mem_region_attributes)	|
> > ]
> > 
> > Reject the host from specifying a memory access descriptor offset
> > that is different than the size of the memory region header.
> 
> And yet you decide that you want to enforce this assumption. I don't
> understand how you arrive to this conclusion.
> 
> Looking at the spec, it appears that the offset is *designed* to allow
> a gap between the header and the EMAD. Refusing to handle a it seems to be a
> violation of the spec.
> 
> What am I missing?

While the spec allows the gap to be variable (since version 1.1), the
arm ff-a driver places it at a fixed position in:
ffa_mem_region_additional_setup() 
https://elixir.bootlin.com/linux/v7.0/source/drivers/firmware/arm_ffa/driver.c#L671

and makes use of the same assumption in: ffa_mem_desc_offset().
https://elixir.bootlin.com/linux/v7.0/source/include/linux/arm_ffa.h#L448
The later one seems wrong IMO. because we should compute the offset
based on the value stored in ep_mem_offset and not adding it up with
sizeof(struct ffa_mem_region).

Maybe this should be the fix instead and not the one in pKVM ? What do
you think ?

The current implementation in pKVM makes use of the
ffa_mem_desc_offset() to validate the first EMAD. If a compromised host
places an EMAD at a different offset than sizeof(struct ffa_mem_region),
then pKVM will not validate that EMAD.

> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.

Thanks,
Sebastian