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

[Sudeep Holla]

On Wed, Apr 22, 2026 at 01:35:55PM +0000, Sebastian Ene wrote:
> 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@xxxxxxxxxx> 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
> 

That's just the current choice in the driver and can be changed in the future.

> 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

Again this is just in the transmit path of the message the driver is
constructing and hence it is a simple choice rather than wrong assumption.

> 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).
> 

Sorry what am I missing as the driver is building these descriptors to
send it across to SPMC, we are populating the field and it will be 0
before it is initialised

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

Can you share the diff you have in mind to understand your concern better
or are you referring to this patch itself.

> 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.
>

Calling the host as compromised if it chooses a different offset seems bit
of extreme here. I am no sure if I am missing to understand something here.

-- 
Regards,
Sudeep