Re: [PATCH v2] KVM: arm64: Prevent the host from using an smc with imm16 != 0

[Vincent Donnefort <vdonnefort@google.com>]

On Wed, Mar 25, 2026 at 11:31:38AM +0000, Sebastian Ene wrote:
> The ARM Service Calling Convention (SMCCC) specifies that the function
> identifier and parameters should be passed in registers, leaving the
> 16-bit immediate field of the SMC instruction un-handled.
> Currently, our pKVM handler ignores the immediate value, which could lead
> to non-compliant software relying on implementation-defined behavior.
> Enforce the host kernel running under pKVM to use an immediate value
> of 0 by decoding the ISS from the ESR_EL2 and return a not supported
> error code back to the caller.
> 
> Signed-off-by: Sebastian Ene <sebastianene@google.com>
> ---
> v1 -> v2:
> 
>  - Dropped injecting an UNDEF and return an error instead
>    (SMCCC_RET_NOT_SUPPORTED)
>  - Used the mask ESR_ELx_xVC_IMM_MASK instead of masking with U16_MAX
>  - Updated the title of the commit message from:
>    "[PATCH] KVM: arm64: Inject UNDEF when host is executing an
>     smc with imm16 != 0

> ---
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index e7790097db93..4ffe30fd8707 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -762,6 +762,12 @@ void handle_trap(struct kvm_cpu_context *host_ctxt)
>  		handle_host_hcall(host_ctxt);
>  		break;
>  	case ESR_ELx_EC_SMC64:
> +		if (ESR_ELx_xVC_IMM_MASK & esr) {
> +			cpu_reg(host_ctxt, 0) = SMCCC_RET_NOT_SUPPORTED;
> +			kvm_skip_host_instr();
> +			break;
> +		}
> +

I wonder if it isn't better to move that into handle_host_smc() as this is part
of how we handle the SMC after all? (and it calls that kvm_skip_host_instr()
already)

>  		handle_host_smc(host_ctxt);
>  		break;
>  	case ESR_ELx_EC_IABT_LOW:
> -- 
> 2.53.0.1018.g2bb0e51243-goog
>