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

[Mark Rutland <mark.rutland@arm.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.

That's not quite right; the SMCCC spec says callers must use immediate
0.

See https://developer.arm.com/documentation/den0028/h/ section 2.10
("SME and HVC immediate value"), which says:

| • For all compliant calls, an SMC or HVC immediate value of zero must be
|   used.
| • Nonzero immediate values in SMC instructions are reserved.
| • Nonzero immediate values in HVC instructions are designated for use by
|   hypervisor vendors.

> 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.

From my PoV, it'd be fine to turn a non-zero immediate into an UNDEF:

* For HVC, we can say any non-zero immediate represents a request to KVM to
  inject an UNDEF.

* For SMC, the dbehaviour is not defined.

I am also ok with returning a value in x0, BUT that's stronger than SMCCC
actually requires.

Mark.

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