KVM: x86: Protect exit_reason from being used in Spectre-v1/L1TF attacks

This fixes a Spectre-v1/L1TF vulnerability in vmx_handle_exit().
While exit_reason is set by the hardware and therefore should not be
attacker-influenced, an unknown exit_reason could potentially be used to
perform such an attack.

Fixes: 55d2375e ("KVM: nVMX: Move nested code to dedicated files")
Signed-off-by: Marios Pomonis <pomonis@google.com>
Signed-off-by: Nick Finco <nifi@google.com>
Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Andrew Honig <ahonig@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -5913,8 +5913,10 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
 	if (exit_fastpath == EXIT_FASTPATH_SKIP_EMUL_INS) {
 		kvm_skip_emulated_instruction(vcpu);
 		return 1;
-	} else if (exit_reason < kvm_vmx_max_exit_handlers
-	    && kvm_vmx_exit_handlers[exit_reason]) {
+	}
+
+	if (exit_reason >= kvm_vmx_max_exit_handlers)
+		goto unexpected_vmexit;
 #ifdef CONFIG_RETPOLINE
 	if (exit_reason == EXIT_REASON_MSR_WRITE)
 		return kvm_emulate_wrmsr(vcpu);
@@ -5929,10 +5931,16 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
 	else if (exit_reason == EXIT_REASON_EPT_MISCONFIG)
 		return handle_ept_misconfig(vcpu);
 #endif
+
+	exit_reason = array_index_nospec(exit_reason,
+					 kvm_vmx_max_exit_handlers);
+	if (!kvm_vmx_exit_handlers[exit_reason])
+		goto unexpected_vmexit;
+
 	return kvm_vmx_exit_handlers[exit_reason](vcpu);
-	} else {
-		vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
-				exit_reason);
+
+unexpected_vmexit:
+	vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n", exit_reason);
 	dump_vmcs();
 	vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
 	vcpu->run->internal.suberror =
@@ -5940,7 +5948,6 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
 	vcpu->run->internal.ndata = 1;
 	vcpu->run->internal.data[0] = exit_reason;
 	return 0;
-	}
 }
 
 /*