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

Sebastian Ene
2026-04-27T11:36:30Z

On Thu, Apr 23, 2026 at 10:55:34AM +0100, Sudeep Holla wrote:
> On Thu, Apr 23, 2026 at 09:17:49AM +0000, Sebastian Ene wrote:
> > On Wed, Apr 22, 2026 at 08:29:06PM +0100, Sudeep Holla wrote:
>
> [...]
>
> > Hello Sudeep,
> >
> > > 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
> >
> > Right, what I meant is having something like this since this function is not limited
> > to the driver scope and using it from other components would imply relying on the
> > assumption: 'ep_mem_offset == sizeof(struct ffa_mem_region)'. We will also have to validate
> > that the `ep_mem_offset` doesn't point outside of the mailbox designated buffer.
> >
>
> Sure, we can extend the function itself or add addition helper to get the
> functionality you are looking for the validation.
>

Thanks, would it be ok to BUG_ON if the offset is out of range here ?
(we would probably have to pass the size of the buf as well in this
function)

> > ---
> > diff --git a/include/linux/arm_ffa.h b/include/linux/arm_ffa.h
> > index 81e603839c4a..62d67dae8b70 100644
> > --- a/include/linux/arm_ffa.h
> > +++ b/include/linux/arm_ffa.h
> > @@ -445,7 +445,7 @@ ffa_mem_desc_offset(struct ffa_mem_region *buf, int count, u32 ffa_version)
> > if (!FFA_MEM_REGION_HAS_EP_MEM_OFFSET(ffa_version))
> > offset += offsetof(struct ffa_mem_region, ep_mem_offset);
> > else
> > - offset += sizeof(struct ffa_mem_region);
> > + offset += buf->ep_mem_offset;
> >
> > return offset;
> > }
> > ---
> >
> > And then move `ffa_mem_region_additional_setup` to be called earlier before `ffa_mem_desc_offset`:
> > (so that it can setup the value for ep_mem_offset)
> >
> > ---
> > diff --git a/drivers/firmware/arm_ffa/driver.c b/drivers/firmware/arm_ffa/driver.c
> > index f2f94d4d533e..66de59c88aff 100644
> > --- a/drivers/firmware/arm_ffa/driver.c
> > +++ b/drivers/firmware/arm_ffa/driver.c
> > @@ -691,6 +691,8 @@ ffa_setup_and_transmit(u32 func_id, void *buffer, u32 max_fragsize,
> > mem_region->flags = args->flags;
> > mem_region->sender_id = drv_info->vm_id;
> > mem_region->attributes = ffa_memory_attributes_get(func_id);
> > +
> > + ffa_mem_region_additional_setup(drv_info->version, mem_region);
>
> Ah this could do the trick. I need to check if all the usages are covered
> though.
>

I looked a bit at the call paths and I think we can use it like this.
Please let me know if you found it differently. I would like to re-spin
another version of this patch.

> > composite_offset = ffa_mem_desc_offset(buffer, args->nattrs,
> > drv_info->version);
> >
> > @@ -708,7 +710,6 @@ ffa_setup_and_transmit(u32 func_id, void *buffer, u32 max_fragsize,
> > }
> > mem_region->handle = 0;
> > mem_region->ep_count = args->nattrs;
> > - ffa_mem_region_additional_setup(drv_info->version, mem_region);
> > ---
> >
> > >
> > > > 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.
> >
> > Sure, please let me know if you think this is wrong. I might have misunderstood it.
> >
>
> Nope, the patch helped to understand it quicker. Thanks for that.
>
> > >
> > > > 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.
> > >
> >
> > Sorry for not explaining it, in pKVM model we don't trust the host kernel so
> > we can assume that everything that doesn't pass the hypervisor validation(in
> > this case the ff-a memory transaction) can be a potential attack that wants
> > to compromise EL2.
> >
>
> I am aware of the principle in general, but this example with different offset
> can't be assumed as comprised host if the offset + size is well within the
> Tx buffer size boundaries. That should be the way for you to cross check for
> any compromise IHMO.
>

I agree, it cannot be assumed as a compromised host it can be perferctly
normal with another driver that places it at a different offset; that's
why I suggested patching ffa_mem_desc_offset instead and doing the
ep_mem_offset validation there.

> --
> Regards,
> Sudeep

Thanks,
Sebastian