Code: static void init_vmcb(struct vcpu_svm *svm)
{
	struct vmcb_control_area *control = &svm->vmcb->control;
	struct vmcb_save_area *save = &svm->vmcb->save;

	svm->vcpu.fpu_active = 1;
	svm->vcpu.arch.hflags = 0;

	set_cr_intercept(svm, INTERCEPT_CR0_READ);
	set_cr_intercept(svm, INTERCEPT_CR3_READ);
	set_cr_intercept(svm, INTERCEPT_CR4_READ);
	set_cr_intercept(svm, INTERCEPT_CR0_WRITE);
	set_cr_intercept(svm, INTERCEPT_CR3_WRITE);
	set_cr_intercept(svm, INTERCEPT_CR4_WRITE);
	set_cr_intercept(svm, INTERCEPT_CR8_WRITE);

	set_dr_intercepts(svm);

 	set_exception_intercept(svm, PF_VECTOR);
 	set_exception_intercept(svm, UD_VECTOR);
 	set_exception_intercept(svm, MC_VECTOR);
 
 	set_intercept(svm, INTERCEPT_INTR);
 	set_intercept(svm, INTERCEPT_NMI);
	set_intercept(svm, INTERCEPT_SMI);
	set_intercept(svm, INTERCEPT_SELECTIVE_CR0);
	set_intercept(svm, INTERCEPT_RDPMC);
	set_intercept(svm, INTERCEPT_CPUID);
	set_intercept(svm, INTERCEPT_INVD);
	set_intercept(svm, INTERCEPT_HLT);
	set_intercept(svm, INTERCEPT_INVLPG);
	set_intercept(svm, INTERCEPT_INVLPGA);
	set_intercept(svm, INTERCEPT_IOIO_PROT);
	set_intercept(svm, INTERCEPT_MSR_PROT);
	set_intercept(svm, INTERCEPT_TASK_SWITCH);
	set_intercept(svm, INTERCEPT_SHUTDOWN);
	set_intercept(svm, INTERCEPT_VMRUN);
	set_intercept(svm, INTERCEPT_VMMCALL);
	set_intercept(svm, INTERCEPT_VMLOAD);
	set_intercept(svm, INTERCEPT_VMSAVE);
	set_intercept(svm, INTERCEPT_STGI);
	set_intercept(svm, INTERCEPT_CLGI);
	set_intercept(svm, INTERCEPT_SKINIT);
	set_intercept(svm, INTERCEPT_WBINVD);
	set_intercept(svm, INTERCEPT_MONITOR);
	set_intercept(svm, INTERCEPT_MWAIT);
	set_intercept(svm, INTERCEPT_XSETBV);

	control->iopm_base_pa = iopm_base;
	control->msrpm_base_pa = __pa(svm->msrpm);
	control->int_ctl = V_INTR_MASKING_MASK;

	init_seg(&save->es);
	init_seg(&save->ss);
	init_seg(&save->ds);
	init_seg(&save->fs);
	init_seg(&save->gs);

	save->cs.selector = 0xf000;
	save->cs.base = 0xffff0000;
	/* Executable/Readable Code Segment */
	save->cs.attrib = SVM_SELECTOR_READ_MASK | SVM_SELECTOR_P_MASK |
		SVM_SELECTOR_S_MASK | SVM_SELECTOR_CODE_MASK;
	save->cs.limit = 0xffff;

	save->gdtr.limit = 0xffff;
	save->idtr.limit = 0xffff;

	init_sys_seg(&save->ldtr, SEG_TYPE_LDT);
	init_sys_seg(&save->tr, SEG_TYPE_BUSY_TSS16);

	svm_set_efer(&svm->vcpu, 0);
	save->dr6 = 0xffff0ff0;
	kvm_set_rflags(&svm->vcpu, 2);
	save->rip = 0x0000fff0;
	svm->vcpu.arch.regs[VCPU_REGS_RIP] = save->rip;

	/*
	 * svm_set_cr0() sets PG and WP and clears NW and CD on save->cr0.
	 * It also updates the guest-visible cr0 value.
	 */
	svm_set_cr0(&svm->vcpu, X86_CR0_NW | X86_CR0_CD | X86_CR0_ET);
	kvm_mmu_reset_context(&svm->vcpu);

	save->cr4 = X86_CR4_PAE;
	/* rdx = ?? */

	if (npt_enabled) {
		/* Setup VMCB for Nested Paging */
		control->nested_ctl = 1;
		clr_intercept(svm, INTERCEPT_INVLPG);
		clr_exception_intercept(svm, PF_VECTOR);
		clr_cr_intercept(svm, INTERCEPT_CR3_READ);
		clr_cr_intercept(svm, INTERCEPT_CR3_WRITE);
		save->g_pat = svm->vcpu.arch.pat;
		save->cr3 = 0;
		save->cr4 = 0;
	}
	svm->asid_generation = 0;

	svm->nested.vmcb = 0;
	svm->vcpu.arch.hflags = 0;

	if (boot_cpu_has(X86_FEATURE_PAUSEFILTER)) {
		control->pause_filter_count = 3000;
		set_intercept(svm, INTERCEPT_PAUSE);
	}

	mark_all_dirty(svm->vmcb);

	enable_gif(svm);
}

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>