Re: [PATCH 3/7] KVM: arm64: Validate the host-provided vgic model in pKVM

[Joey Gouly <joey.gouly@arm.com>]

Question(s)
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On Tue, Sep 15, 2026 at 01:38:42PM +0100, Fuad Tabba wrote:
> EL2 copies vgic_model from the host's struct kvm unchecked, and the nVHE
> world switch dispatches on it with no cpucap guard. A host writing
> KVM_DEV_TYPE_ARM_VGIC_V5 makes EL2 access GICv5 CPU interface registers
> that are UNDEFINED without FEAT_GCIE, and panics the hypervisor on any
> GICv3 machine.
> 
> Accept only GICv3, the one model pKVM supports, forcing anything else
> to 0.
> 
> Fixes: 9b8e3d4ca0e73 ("KVM: arm64: gic-v5: Implement GICv5 load/put and save/restore")
> Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
> ---
>  arch/arm64/kvm/hyp/nvhe/pkvm.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> index 459bd9eb7e4bc..4c33c863e90e1 100644
> --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> @@ -340,13 +340,19 @@ static void pkvm_init_features_from_host(struct pkvm_hyp_vm *hyp_vm, const struc
>  {
>  	struct kvm *kvm = &hyp_vm->kvm;
>  	unsigned long host_arch_flags = READ_ONCE(host_kvm->arch.flags);
> +	u32 vgic_model = READ_ONCE(host_kvm->arch.vgic.vgic_model);
>  	DECLARE_BITMAP(allowed_features, KVM_VCPU_MAX_FEATURES);
>  
>  	/* CTR_EL0 is always under host control, even for protected VMs. */
>  	hyp_vm->kvm.arch.ctr_el0 = host_kvm->arch.ctr_el0;
>  
> -	/* Preserve the vgic model so that GICv3 emulation works */
> -	hyp_vm->kvm.arch.vgic.vgic_model = host_kvm->arch.vgic.vgic_model;
> +	/*
> +	 * GICv3 is the only model pKVM runs, and the GICv5 world switch
> +	 * touches registers UNDEFINED at EL2 without FEAT_GCIE.
> +	 */
> +	if (vgic_model != KVM_DEV_TYPE_ARM_VGIC_V3)
> +		vgic_model = 0;
> +	hyp_vm->kvm.arch.vgic.vgic_model = vgic_model;

A bit confused by what is expected by writing 0 here? I would have maybe
expected to refuse running this VM? What happens if the host writes some
garbage here, so vgic_model=0, but then tries to access gic-v3
registers?

Thanks,
Joey

>  
>  	/* No restrictions for non-protected VMs. */
>  	if (!kvm_vm_is_protected(kvm)) {
> -- 
> 2.39.5
>