Re: [PATCH v5 1/7] KVM: arm64: Enforce strict SBZ checks in the FF-A proxy

[Will Deacon]

Hi all,

On Tue, Jun 23, 2026 at 11:53:48AM +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 | 54 +++++++++++++++++++++++++++++++++++
>  1 file changed, 54 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/ffa.c b/arch/arm64/kvm/hyp/nvhe/ffa.c
> index 1af722771178..78bb043b33ee 100644
> --- a/arch/arm64/kvm/hyp/nvhe/ffa.c
> +++ b/arch/arm64/kvm/hyp/nvhe/ffa.c
> @@ -71,6 +71,20 @@ 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;
> +
> +	end_reg = ARM_SMCCC_IS_64(func_id) ? 17 : 7;
> +	for (reg = first_reg; reg <= end_reg; reg++) {
> +		if (cpu_reg(ctxt, reg))
> +			return true;
> +	}
> +
> +	return false;
> +}

Seb and I tried taking this for a spin on some Android devices and, sadly,
it leads to fireworks. The reason is that the FF-A spec quietly changed
the list of unused parameter registers for 64-bit SMCs from v1.1 to v1.2
of the spec so that pre-existing calls were affected.

For example, in v1.1 a 64-bit RXTX_MAP only has x4-x7 as MBZ, whereas in
v1.2 the same call has x4-x17 as SBZ.

We can follow the spec by predicating the additional check on the FF-A
version being >= 1.2, but I'm not hopeful that existing drivers are
compliant. I also suggest moving this patch to the end of the series in
case we need to revert it.

Cheers,

Will