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>
static int handle_vmptrld(struct kvm_vcpu *vcpu)
{
	struct vcpu_vmx *vmx = to_vmx(vcpu);
	gpa_t vmptr;

	if (!nested_vmx_check_permission(vcpu))
		return 1;

	if (nested_vmx_check_vmptr(vcpu, EXIT_REASON_VMPTRLD, &vmptr))
		return 1;

	if (vmx->nested.current_vmptr != vmptr) {
		struct vmcs12 *new_vmcs12;
		struct page *page;
		page = nested_get_page(vcpu, vmptr);
		if (page == NULL) {
			nested_vmx_failInvalid(vcpu);
			skip_emulated_instruction(vcpu);
			return 1;
		}
		new_vmcs12 = kmap(page);
		if (new_vmcs12->revision_id != VMCS12_REVISION) {
			kunmap(page);
			nested_release_page_clean(page);
			nested_vmx_failValid(vcpu,
				VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID);
			skip_emulated_instruction(vcpu);
			return 1;
		}

		nested_release_vmcs12(vmx);
		vmx->nested.current_vmptr = vmptr;
		vmx->nested.current_vmcs12 = new_vmcs12;
		vmx->nested.current_vmcs12_page = page;
		if (enable_shadow_vmcs) {
			vmcs_set_bits(SECONDARY_VM_EXEC_CONTROL,
				      SECONDARY_EXEC_SHADOW_VMCS);
			vmcs_write64(VMCS_LINK_POINTER,
				     __pa(vmx->nested.current_shadow_vmcs));
			vmx->nested.sync_shadow_vmcs = true;
		}
	}

	nested_vmx_succeed(vcpu);
	skip_emulated_instruction(vcpu);
	return 1;
}

static int handle_vmptrld(struct kvm_vcpu *vcpu)
{
	struct vcpu_vmx *vmx = to_vmx(vcpu);
	gpa_t vmptr;

	if (!nested_vmx_check_permission(vcpu))
		return 1;

	if (nested_vmx_check_vmptr(vcpu, EXIT_REASON_VMPTRLD, &vmptr))
		return 1;

	if (vmx->nested.current_vmptr != vmptr) {
		struct vmcs12 *new_vmcs12;
		struct page *page;
		page = nested_get_page(vcpu, vmptr);
		if (page == NULL) {
			nested_vmx_failInvalid(vcpu);
			skip_emulated_instruction(vcpu);
			return 1;
		}
		new_vmcs12 = kmap(page);
		if (new_vmcs12->revision_id != VMCS12_REVISION) {
			kunmap(page);
			nested_release_page_clean(page);
			nested_vmx_failValid(vcpu,
				VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID);
			skip_emulated_instruction(vcpu);
			return 1;
		}

		nested_release_vmcs12(vmx);
		vmx->nested.current_vmptr = vmptr;
		vmx->nested.current_vmcs12 = new_vmcs12;
		vmx->nested.current_vmcs12_page = page;
		if (enable_shadow_vmcs) {
			vmcs_set_bits(SECONDARY_VM_EXEC_CONTROL,
				      SECONDARY_EXEC_SHADOW_VMCS);
			vmcs_write64(VMCS_LINK_POINTER,
				     __pa(vmx->nested.current_shadow_vmcs));
			vmx->nested.sync_shadow_vmcs = true;
		}
	}

	nested_vmx_succeed(vcpu);
	skip_emulated_instruction(vcpu);
	return 1;
}
