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

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 <[e-mail]>
Cc: [e-mail]
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/include/uapi/asm/svm.h

···
 	{ 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/svm.c

···
 	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);
···
 	er = emulate_instruction(&svm->vcpu, EMULTYPE_TRAP_UD);
 	if (er != EMULATE_DONE)
 		kvm_queue_exception(&svm->vcpu, UD_VECTOR);
+	return 1;
+}
+
+static int ac_interception(struct vcpu_svm *svm)
+{
+	kvm_queue_exception_e(&svm->vcpu, AC_VECTOR, 0);
 	return 1;
 }
 
···
 	[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/vmx.c

···
 	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))
···
 		return handle_rmode_exception(vcpu, ex_no, error_code);
 
 	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 &