Code: static int handle_exception(struct kvm_vcpu *vcpu)
{
	struct vcpu_vmx *vmx = to_vmx(vcpu);
	struct kvm_run *kvm_run = vcpu->run;
	u32 intr_info, ex_no, error_code;
	unsigned long cr2, rip, dr6;
	u32 vect_info;
	enum emulation_result er;

	vect_info = vmx->idt_vectoring_info;
	intr_info = vmx->exit_intr_info;

	if (is_machine_check(intr_info))
		return handle_machine_check(vcpu);

	if ((intr_info & INTR_INFO_INTR_TYPE_MASK) == INTR_TYPE_NMI_INTR)
		return 1;  /* already handled by vmx_vcpu_run() */

	if (is_no_device(intr_info)) {
		vmx_fpu_activate(vcpu);
		return 1;
	}

	if (is_invalid_opcode(intr_info)) {
		if (is_guest_mode(vcpu)) {
			kvm_queue_exception(vcpu, UD_VECTOR);
			return 1;
		}
		er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
		if (er != EMULATE_DONE)
			kvm_queue_exception(vcpu, UD_VECTOR);
		return 1;
	}

	error_code = 0;
	if (intr_info & INTR_INFO_DELIVER_CODE_MASK)
		error_code = vmcs_read32(VM_EXIT_INTR_ERROR_CODE);

	/*
	 * The #PF with PFEC.RSVD = 1 indicates the guest is accessing
	 * MMIO, it is better to report an internal error.
	 * See the comments in vmx_handle_exit.
	 */
	if ((vect_info & VECTORING_INFO_VALID_MASK) &&
	    !(is_page_fault(intr_info) && !(error_code & PFERR_RSVD_MASK))) {
		vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
		vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_SIMUL_EX;
		vcpu->run->internal.ndata = 3;
		vcpu->run->internal.data[0] = vect_info;
		vcpu->run->internal.data[1] = intr_info;
		vcpu->run->internal.data[2] = error_code;
		return 0;
	}

	if (is_page_fault(intr_info)) {
		/* EPT won't cause page fault directly */
		BUG_ON(enable_ept);
		cr2 = vmcs_readl(EXIT_QUALIFICATION);
		trace_kvm_page_fault(cr2, error_code);

		if (kvm_event_needs_reinjection(vcpu))
			kvm_mmu_unprotect_page_virt(vcpu, cr2);
		return kvm_mmu_page_fault(vcpu, cr2, error_code, NULL, 0);
	}

	ex_no = intr_info & INTR_INFO_VECTOR_MASK;

	if (vmx->rmode.vm86_active && rmode_exception(vcpu, ex_no))
 		return handle_rmode_exception(vcpu, ex_no, error_code);
 
 	switch (ex_no) {
 	case DB_VECTOR:
 		dr6 = vmcs_readl(EXIT_QUALIFICATION);
 		if (!(vcpu->guest_debug &
		      (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))) {
			vcpu->arch.dr6 &= ~15;
			vcpu->arch.dr6 |= dr6 | DR6_RTM;
			if (!(dr6 & ~DR6_RESERVED)) /* icebp */
				skip_emulated_instruction(vcpu);

			kvm_queue_exception(vcpu, DB_VECTOR);
			return 1;
		}
		kvm_run->debug.arch.dr6 = dr6 | DR6_FIXED_1;
		kvm_run->debug.arch.dr7 = vmcs_readl(GUEST_DR7);
		/* fall through */
	case BP_VECTOR:
		/*
		 * Update instruction length as we may reinject #BP from
		 * user space while in guest debugging mode. Reading it for
		 * #DB as well causes no harm, it is not used in that case.
		 */
		vmx->vcpu.arch.event_exit_inst_len =
			vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
		kvm_run->exit_reason = KVM_EXIT_DEBUG;
		rip = kvm_rip_read(vcpu);
		kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
		kvm_run->debug.arch.exception = ex_no;
		break;
	default:
		kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
		kvm_run->ex.exception = ex_no;
		kvm_run->ex.error_code = error_code;
		break;
	}
	return 0;
}

Vulnerability Type: DoS 
CWE ID: CWE-399
Summary: The KVM subsystem in the Linux kernel through 4.2.6, and Xen 4.3.x through 4.6.x, allows guest OS users to cause a denial of service (host OS panic or hang) by triggering many #AC (aka Alignment Check) exceptions, related to svm.c and vmx.c.
Commit Message: 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 <digitaleric@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>