Re: [PATCH v6 7/7] KVM: arm64: Enforce strict SBZ checks in the FF-A proxy

[Will Deacon]

On Fri, Jun 26, 2026 at 09:24:52AM +0000, Sebastian Ene wrote:
> On Fri, Jun 26, 2026 at 10:11:14AM +0100, Will Deacon wrote:
> > On Fri, Jun 26, 2026 at 07:45:45AM +0000, Sebastian Ene wrote:
> > > Introduce a helper method ffa_check_unused_args_sbz to enforce strict
> > > arguments checking when the hypervisor acts as a relayer between the
> > > host and Trustzone.
> > > 
> > > Signed-off-by: Sebastian Ene <sebastianene@xxxxxxxxxx>
> > > Reviewed-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
> > > ---
> > >  arch/arm64/kvm/hyp/nvhe/ffa.c | 96 ++++++++++++++++++++++++++++++++++-
> > >  1 file changed, 95 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > > index 712811e89435..bd50ddc5b61c 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> > > @@ -74,6 +74,21 @@ static u32 hyp_ffa_version;
> > >  static bool has_version_negotiated;
> > >  static hyp_spinlock_t version_lock;
> > >  
> > > +static bool ffa_check_unused_args_sbz(struct kvm_cpu_context *ctxt, int first_reg)
> > > +{
> > > +	DECLARE_REG(u32, func_id, ctxt, 0);
> > > +	int reg, end_reg = 7;
> > > +
> > > +	if (FFA_MINOR_VERSION(hyp_ffa_version) >= 2)
> > > +		end_reg = ARM_SMCCC_IS_64(func_id) ? 17 : 7;
> > 
> > This looks like an accident waiting to happen if we don't check the major
> > number as well.
> > 
> > I think you should just check:
> > 
> > 	if (hyp_ffa_version >= FFA_VERSION_1_2)
> > 
> > instead.
> > 
> > You should also add a comment.
> 
> We restrict hyp_ffa_version major to 1.0 here
> https://elixir.bootlin.com/linux/v7.1.1/source/arch/arm64/kvm/hyp/nvhe/ffa.c#L962
> but since this is an easy fix I will include it.

Thanks, Seb. I'm just certain that if/when we update that check, we'll
forget to update this bit at the same time (I know I would!). So doing
it now is the simplest thing.

With that change:

Acked-by: Will Deacon <will@xxxxxxxxxx>

Will