Re: [PATCH v1 1/4] KVM: arm64: Validate the host-provided vgic model in pKVM

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [Critical] Unconditional copy of the host's mdcr_el2 to the protected VM's mdcr_el2 allows the host to reconfigure the guest's secure trap environment.
- [Medium] The patch fails to sanitize the explicit `__vgic_v5_save_apr` and `__vgic_v5_restore_vmcr_apr` hypercalls, allowing a malicious host to still panic the hypervisor on GICv3 machines.
--

--- Patch [1]: [PATCH v1 1/4] KVM: arm64: Validate the host-provided vgic model in pKVM ---
commit 3364f007874647c1650c3fc747633d9c5c9b87de
Author: Fuad Tabba <fuad.tabba@linux.dev>

KVM: arm64: Validate the host-provided vgic model in pKVM

This commit validates the host-provided vgic_model during initialization
of a protected VM, ensuring only GICv2 and GICv3 models are permitted.
This prevents a malicious host from causing a hypervisor panic by setting
the model to GICv5 on a GICv3 machine.

> diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> index 24d6f164129ac..59bb15efdca42 100644
> --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> @@ -340,13 +340,25 @@ static void pkvm_init_features_from_host(struct pkvm_hyp_vm *hyp_vm, const struc
>  {

[ ... ]

> -	/* Preserve the vgic model so that GICv3 emulation works */
> -	hyp_vm->kvm.arch.vgic.vgic_model = host_kvm->arch.vgic.vgic_model;
> +	/*
> +	 * Preserve the vgic model for GICv3 emulation, but only what pKVM can
> +	 * run: the GICv5 world switch touches registers UNDEFINED at EL2
> +	 * without FEAT_GCIE. 0 is not a valid kvm_device_type: "no vgic".
> +	 */
> +	switch (vgic_model) {
> +	case KVM_DEV_TYPE_ARM_VGIC_V2:
> +	case KVM_DEV_TYPE_ARM_VGIC_V3:
> +		break;
> +	default:
> +		vgic_model = 0;
> +	}
> +	hyp_vm->kvm.arch.vgic.vgic_model = vgic_model;

[Severity: Medium]
This is a pre-existing issue, but does this validation fully prevent a
malicious host from triggering a GICv5 panic?

Even with the model restricted here, the explicit GICv5 hypercalls
remain unconditionally exposed in the always-on band:

arch/arm64/kvm/hyp/nvhe/hyp-main.c:
static void handle___vgic_v5_save_apr(struct kvm_cpu_context *host_ctxt)
{
	DECLARE_REG(struct vgic_v5_cpu_if *, cpu_if, host_ctxt, 1);

	__vgic_v5_save_apr(kern_hyp_va(cpu_if));
}

static void handle___vgic_v5_restore_vmcr_apr(struct kvm_cpu_context *host_ctxt)
{
	DECLARE_REG(struct vgic_v5_cpu_if *, cpu_if, host_ctxt, 1);

	__vgic_v5_restore_vmcr_apr(kern_hyp_va(cpu_if));
}

Can a compromised host still bypass this vgic_model sanitization by
directly executing the HVC instruction for
__KVM_HOST_SMCCC_FUNC___vgic_v5_save_apr or __vgic_v5_restore_vmcr_apr?

If these directly access GICv5 registers, wouldn't this still trigger an
UNDEFINED exception on machines without FEAT_GCIE?

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

[ ... ]

Additionally, while reviewing the protected guest state handling, I noticed
another potential issue in the hypervisor main loop:

arch/arm64/kvm/hyp/nvhe/hyp-main.c:flush_hyp_vcpu() {
	...
	/* __hyp_running_vcpu must be NULL in a guest context. */
	hyp_vcpu->vcpu.arch.ctxt.__hyp_running_vcpu = NULL;

	hyp_vcpu->vcpu.arch.mdcr_el2	= host_vcpu->arch.mdcr_el2;
	/*
	 * HCR_EL2.VSE is host-owned (a pending virtual SError to inject), not a
	...
}

[Severity: Critical]
This isn't a bug introduced by this patch, but does this unconditional
assignment allow the untrusted host to clobber the securely initialized
MDCR_EL2 configuration of protected guests on every VCPU entry?

Since flush_hyp_vcpu() overwrites the hyp vCPU's mdcr_el2 with the
host-controlled host_vcpu->arch.mdcr_el2, could a compromised host use
this to reconfigure the performance monitor (TPM) and debug (TDA/TDE)
trap environment of a protected guest?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260806100256.371164-1-fuad.tabba@linux.dev?part=1