Subject: Re: [PATCH] KVM: arm64: Inject UNDEF when host is executing an smc with imm16 != 0

From: Vincent Donnefort <[email-masked]>
Date: Tue, 24 Mar 2026 14:06:40 +0000

On Tue, Mar 24, 2026 at 01:57:28PM +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 inject an undefined
> instruction exception back to the caller.
> 
> Signed-off-by: Sebastian Ene <[email-masked]>
> ---
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index e7790097db93..ff6a90a4a4c7 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -756,13 +756,18 @@ static bool handle_host_mte(u64 esr)
>  void handle_trap(struct kvm_cpu_context *host_ctxt)
>  {
>  	u64 esr = read_sysreg_el2(SYS_ESR);
> +	u16 imm16;
>  
>  	switch (ESR_ELx_EC(esr)) {
>  	case ESR_ELx_EC_HVC64:
>  		handle_host_hcall(host_ctxt);
>  		break;
>  	case ESR_ELx_EC_SMC64:
> -		handle_host_smc(host_ctxt);
> +		imm16 = esr & U16_MAX;
> +		if (!imm16)

if (ESR_ELx_xVC_IMM_MASK(esr)) ?

Also, I can't find void inject_undef64(void); I think you need a vcpu for that?

> +			handle_host_smc(host_ctxt);
> +		else
> +			inject_undef64();
>  		break;
>  	case ESR_ELx_EC_IABT_LOW:
>  	case ESR_ELx_EC_DABT_LOW:
> -- 
> 2.53.0.983.g0bb29b3bc5-goog
>