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

Sebastian Ene
2026-04-23T09:29:12Z

On Thu, Apr 23, 2026 at 09:08:46AM +0100, Marc Zyngier wrote:
> On Wed, 22 Apr 2026 14:35:55 +0100,
> Sebastian Ene <sebastianene@google.com> 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@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
>
> That's an implementation detail, and you shouldn't rely on this.
>
> > 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 ?
>
> I think you should parse the buffers as the spec intends them, without
> assumptions or limitations.

Ack.

>
> >
> > 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.
>
> Why compromised? Isn't that a perfectly valid thing to do? What I
> understand is that the FFA 1.1 implementation in pKVM doesn't match
> the expectations of the spec. If that's indeed the case, pKVM should
> be fixed to accept these messages correctly, or stop using FFA 1.1.
>
> M.

Sorry, what I meant is that a potentially malicious host could abuse
this limitation of the FF-A proxy validation which is looking at a fixed
offset to do the EMAD validation. Another EMAD can be placed at a
different offset and it will bypass the validation of the proxy
alltogether.

We have two choices: the simple one is what this patch does (enforce a
fixed offset) or the second one : patch `ffa_mem_desc_offset` to use
ep_mem_offset instead of `sizeof(struct ffa_mem_region)` and validate
the ep_mem_offset.

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

Thanks,
Sebastian