KVM: x86: work around infinite loop in microcode when #AC is delivered

commit 54a20552 upstream.

It was found that a guest can DoS a host by triggering an infinite
stream of "alignment check" (#AC) exceptions.  This causes the
microcode to enter an infinite loop where the core never receives
another interrupt.  The host kernel panics pretty quickly due to the
effects (CVE-2015-5307).
Signed-off-by: Eric Northup <digitaleric@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.2:
 - Add definition of AC_VECTOR
 - Adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/x86/include/asm/kvm_host.h

@@ -83,6 +83,7 @@
 #define GP_VECTOR 13
 #define PF_VECTOR 14
 #define MF_VECTOR 16
+#define AC_VECTOR 17
 #define MC_VECTOR 18
 
 #define SELECTOR_TI_MASK (1 << 2)

arch/x86/kvm/svm.c

@@ -1014,6 +1014,7 @@ static void init_vmcb(struct vcpu_svm *svm)
 	set_exception_intercept(svm, PF_VECTOR);
 	set_exception_intercept(svm, UD_VECTOR);
 	set_exception_intercept(svm, MC_VECTOR);
+	set_exception_intercept(svm, AC_VECTOR);
 
 	set_intercept(svm, INTERCEPT_INTR);
 	set_intercept(svm, INTERCEPT_NMI);
@@ -1689,6 +1690,12 @@ static int ud_interception(struct vcpu_svm *svm)
 	return 1;
 }
 
+static int ac_interception(struct vcpu_svm *svm)
+{
+	kvm_queue_exception_e(&svm->vcpu, AC_VECTOR, 0);
+	return 1;
+}
+
 static void svm_fpu_activate(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
@@ -3188,6 +3195,7 @@ static int (*svm_exit_handlers[])(struct vcpu_svm *svm) = {
 	[SVM_EXIT_EXCP_BASE + PF_VECTOR]	= pf_interception,
 	[SVM_EXIT_EXCP_BASE + NM_VECTOR]	= nm_interception,
 	[SVM_EXIT_EXCP_BASE + MC_VECTOR]	= mc_interception,
+	[SVM_EXIT_EXCP_BASE + AC_VECTOR]	= ac_interception,
 	[SVM_EXIT_INTR]				= intr_interception,
 	[SVM_EXIT_NMI]				= nmi_interception,
 	[SVM_EXIT_SMI]				= nop_on_interception,

arch/x86/kvm/trace.h

@@ -244,6 +244,7 @@ TRACE_EVENT(kvm_apic,
 	{ SVM_EXIT_EXCP_BASE + UD_VECTOR,	"UD excp" }, \
 	{ SVM_EXIT_EXCP_BASE + PF_VECTOR,	"PF excp" }, \
 	{ SVM_EXIT_EXCP_BASE + NM_VECTOR,	"NM excp" }, \
+	{ SVM_EXIT_EXCP_BASE + AC_VECTOR,       "AC excp" }, \
 	{ SVM_EXIT_EXCP_BASE + MC_VECTOR,	"MC excp" }, \
 	{ SVM_EXIT_INTR,			"interrupt" }, \
 	{ SVM_EXIT_NMI,				"nmi" }, \

arch/x86/kvm/vmx.c

@@ -1171,7 +1171,7 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
 	u32 eb;
 
 	eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR) |
-	     (1u << NM_VECTOR) | (1u << DB_VECTOR);
+	     (1u << NM_VECTOR) | (1u << DB_VECTOR) | (1u << AC_VECTOR);
 	if ((vcpu->guest_debug &
 	     (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP)) ==
 	    (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP))
@@ -4266,6 +4266,9 @@ static int handle_exception(struct kvm_vcpu *vcpu)
 
 	ex_no = intr_info & INTR_INFO_VECTOR_MASK;
 	switch (ex_no) {
+	case AC_VECTOR:
+		kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
+		return 1;
 	case DB_VECTOR:
 		dr6 = vmcs_readl(EXIT_QUALIFICATION);
 		if (!(vcpu->guest_debug &