KVM: arm64: Ensure that SME controls are disabled in protected mode

KVM (and pKVM) do not support SME guests. Therefore KVM ensures
that the host's SME state is flushed and that SME controls for
enabling access to ZA storage and for streaming are disabled.

pKVM needs to protect against a buggy/malicious host. Ensure that
it wouldn't run a guest when protected mode is enabled should any
of the SME controls be enabled.
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://lore.kernel.org/r/20240603122852.3923848-10-tabba@google.comSigned-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/fpsimd.c

@@ -90,6 +90,13 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 			fpsimd_save_and_flush_cpu_state();
 		}
 	}
+
+	/*
+	 * If normal guests gain SME support, maintain this behavior for pKVM
+	 * guests, which don't support SME.
+	 */
+	WARN_ON(is_protected_kvm_enabled() && system_supports_sme() &&
+		read_sysreg_s(SYS_SVCR));
 }
 
 /*

arch/arm64/kvm/hyp/nvhe/hyp-main.c

@@ -140,6 +140,17 @@ static void handle___kvm_vcpu_run(struct kvm_cpu_context *host_ctxt)
 		struct pkvm_hyp_vcpu *hyp_vcpu;
 		struct kvm *host_kvm;
 
+		/*
+		 * KVM (and pKVM) doesn't support SME guests for now, and
+		 * ensures that SME features aren't enabled in pstate when
+		 * loading a vcpu. Therefore, if SME features enabled the host
+		 * is misbehaving.
+		 */
+		if (unlikely(system_supports_sme() && read_sysreg_s(SYS_SVCR))) {
+			ret = -EINVAL;
+			goto out;
+		}
+
 		host_kvm = kern_hyp_va(host_vcpu->kvm);
 		hyp_vcpu = pkvm_load_hyp_vcpu(host_kvm->arch.pkvm.handle,
 					      host_vcpu->vcpu_idx);