[PATCH v3 2/4] KVM: arm64: Clear HCR_EL2.RW for 32-bit non-protected vCPUs

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

pKVM keeps its own copy of each vCPU's HCR_EL2 at EL2, initialised with
RW set unconditionally. Nothing stops userspace from creating a
non-protected AArch32 VM, and with RW set, entering one of its vCPUs is
an illegal exception return: KVM_RUN fails with KVM_EXIT_FAIL_ENTRY.

Clear RW for a vCPU that is AArch32 at EL1, when the CPU has AArch32
EL1. On a CPU without it, RW stays set and the entry still fails, rather
than EL2 switching *32_EL2 registers that are UNDEFINED there. The host
already rejects such a vCPU, but EL2 takes the vCPU's features from the
host and doesn't rely on that.

Fixes: b56680de9c648 ("KVM: arm64: Initialize trap register values in hyp in pKVM")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/hyp/nvhe/pkvm.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
index 459bd9eb7e4bc..62d432144d44c 100644
--- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
+++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
@@ -54,6 +54,15 @@ static void pkvm_vcpu_reset_hcr(struct kvm_vcpu *vcpu)
 	else
 		vcpu->arch.hcr_el2 |= HCR_TID2;
 
+	/*
+	 * Without AArch32 EL1, leave RW set and let the entry fail with an
+	 * illegal exception return: the *32_EL2 registers EL2 would otherwise
+	 * switch are UNDEFINED there.
+	 */
+	if (vcpu_has_feature(vcpu, KVM_ARM_VCPU_EL1_32BIT) &&
+	    cpus_have_final_cap(ARM64_HAS_32BIT_EL1))
+		vcpu->arch.hcr_el2 &= ~HCR_EL2_RW;
+
 	if (vcpu_has_ptrauth(vcpu))
 		vcpu->arch.hcr_el2 |= (HCR_API | HCR_APK);
 
-- 
2.39.5