Re: [PATCH 06/18] KVM: arm64: Plug FEAT_GCS handling

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Feb 10, 2025 at 06:41:37PM +0000, Marc Zyngier wrote:
> We don't seem to be handling the GCS-specific exception class.
> Handle it by delivering an UNDEF to the guest, and populate the
> relevant trap bits.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/handle_exit.c | 11 +++++++++++
>  arch/arm64/kvm/sys_regs.c    |  8 ++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 4f8354bf7dc5f..624a78a99e38a 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -294,6 +294,16 @@ static int handle_svc(struct kvm_vcpu *vcpu)
>  	return 1;
>  }
>  
> +static int kvm_handle_gcs(struct kvm_vcpu *vcpu)
> +{
> +	/* We don't expect GCS, so treat it with contempt */
> +	if (kvm_has_feat(vcpu->kvm, ID_AA64PFR1_EL1, GCS, IMP))
> +		WARN_ON_ONCE(1);

Just to check / better my understanging, do we enforce that this can't
be exposed to the guest somewhere?

I see __kvm_read_sanitised_id_reg() masks it out, and the sys_reg_descs
table has it filtered, but I'm not immediately sure whether that
prevents host userspace maliciously setting this?

Otherwise this looks good to me.

Mark.

> +
> +	kvm_inject_undefined(vcpu);
> +	return 1;
> +}
> +
>  static int handle_ls64b(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm *kvm = vcpu->kvm;
> @@ -384,6 +394,7 @@ static exit_handle_fn arm_exit_handlers[] = {
>  	[ESR_ELx_EC_BRK64]	= kvm_handle_guest_debug,
>  	[ESR_ELx_EC_FP_ASIMD]	= kvm_handle_fpasimd,
>  	[ESR_ELx_EC_PAC]	= kvm_handle_ptrauth,
> +	[ESR_ELx_EC_GCS]	= kvm_handle_gcs,
>  };
>  
>  static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 18721c773475d..2ecd0d51a2dae 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -5056,6 +5056,14 @@ void kvm_calculate_traps(struct kvm_vcpu *vcpu)
>  						HFGITR_EL2_nBRBIALL);
>  	}
>  
> +	if (!kvm_has_feat(kvm, ID_AA64PFR1_EL1, GCS, IMP)) {
> +		kvm->arch.fgu[HFGxTR_GROUP] |= (HFGxTR_EL2_nGCS_EL0 |
> +						HFGxTR_EL2_nGCS_EL1);
> +		kvm->arch.fgu[HFGITR_GROUP] |= (HFGITR_EL2_nGCSPUSHM_EL1 |
> +						HFGITR_EL2_nGCSSTR_EL1 |
> +						HFGITR_EL2_nGCSEPP);
> +	}
> +
>  	set_bit(KVM_ARCH_FLAG_FGU_INITIALIZED, &kvm->arch.flags);
>  out:
>  	mutex_unlock(&kvm->arch.config_lock);
> -- 
> 2.39.2
>