KVM: arm64: Filter out HCR_EL2 bits when running in hypervisor context

commit 303084ad12767db64c84ba8fcd0450aec38c8534 upstream.

Most HCR_EL2 bits are not supposed to affect EL2 at all, but only
the guest. However, we gladly merge these bits with the host's
HCR_EL2 configuration, irrespective of entering L1 or L2.

This leads to some funky behaviour, such as L1 trying to inject
a virtual SError for L2, and getting a taste of its own medecine.
Not quite what the architecture anticipated.

In the end, the only bits that matter are those we have defined as
invariants, either because we've made them RESx (E2H, HCD...), or
that we actively refuse to merge because the mess with KVM's own
logic.

Use the sanitisation infrastructure to get the RES1 bits, and let
things rip in a safer way.

Fixes: 04ab519bb86df ("KVM: arm64: nv: Configure HCR_EL2 for FEAT_NV2")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250721101955.535159-3-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 12 insertions, 2 deletions

diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index 477f1580ffea..e482181c6632 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -48,8 +48,7 @@ DEFINE_PER_CPU(unsigned long, kvm_hyp_vector);
 
 static u64 __compute_hcr(struct kvm_vcpu *vcpu)
 {
-	u64 guest_hcr = __vcpu_sys_reg(vcpu, HCR_EL2);
-	u64 hcr = vcpu->arch.hcr_el2;
+	u64 guest_hcr, hcr = vcpu->arch.hcr_el2;
 
 	if (!vcpu_has_nv(vcpu))
 		return hcr;
@@ -68,10 +67,21 @@ static u64 __compute_hcr(struct kvm_vcpu *vcpu)
 		if (!vcpu_el2_e2h_is_set(vcpu))
 			hcr |= HCR_NV1;
 
+		/*
+		 * Nothing in HCR_EL2 should impact running in hypervisor
+		 * context, apart from bits we have defined as RESx (E2H,
+		 * HCD and co), or that cannot be set directly (the EXCLUDE
+		 * bits). Given that we OR the guest's view with the host's,
+		 * we can use the 0 value as the starting point, and only
+		 * use the config-driven RES1 bits.
+		 */
+		guest_hcr = kvm_vcpu_apply_reg_masks(vcpu, HCR_EL2, 0);
+
 		write_sysreg_s(vcpu->arch.ctxt.vncr_array, SYS_VNCR_EL2);
 	} else {
 		host_data_clear_flag(VCPU_IN_HYP_CONTEXT);
 
+		guest_hcr = __vcpu_sys_reg(vcpu, HCR_EL2);
 		if (guest_hcr & HCR_NV) {
 			u64 va = __fix_to_virt(vncr_fixmap(smp_processor_id()));