[PATCH v3 3/5] KVM: arm64: Validate the host-provided vgic model in pKVM

[Fuad Tabba <fuad.tabba@linux.dev>]

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, and force anything else
to 0, as for a VM with no in-kernel irqchip. EL2 reads the model only to
pick the GICv5 world switch and to emulate GICv3 CPU interface accesses,
and the host's own copy is left alone.

Fixes: 9b8e3d4ca0e73 ("KVM: arm64: gic-v5: Implement GICv5 load/put and save/restore")
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
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..a9d0e94709199 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;
+	/*
+	 * pKVM runs only GICv3. Anything else becomes 0, the model of a VM
+	 * with no in-kernel irqchip, which keeps EL2 off the GICv5 registers.
+	 */
+	if (vgic_model != KVM_DEV_TYPE_ARM_VGIC_V3)
+		vgic_model = 0;
+	hyp_vm->kvm.arch.vgic.vgic_model = vgic_model;
 
 	/* No restrictions for non-protected VMs. */
 	if (!kvm_vm_is_protected(kvm)) {
-- 
2.39.5