Commit message: KVM: nSVM: always intercept VMLOAD/VMSAVE when nested (CVE-2021-3656)

If L1 disables VMLOAD/VMSAVE intercepts, and doesn't enable
Virtual VMLOAD/VMSAVE (currently not supported for the nested hypervisor),
then VMLOAD/VMSAVE must operate on the L1 physical memory, which is only
possible by making L0 intercept these instructions.

Failure to do so allowed the nested guest to run VMLOAD/VMSAVE unintercepted,
and thus read/write portions of the host physical memory.

Fixes: 89c8a4984fc9 ("KVM: SVM: Enable Virtual VMLOAD VMSAVE feature")

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
CVE: CVE-2021-3656
CWE: CWE-862
Vulnerability description: A flaw was found in the KVM's AMD code for supporting SVM nested virtualization. The flaw occurs when processing the VMCB (virtual machine control block) provided by the L1 guest to spawn/handle a nested guest (L2). Due to improper validation of the "virt_ext" field, this issue could allow a malicious L1 to disable both VMLOAD/VMSAVE intercepts and VLS (Virtual VMLOAD/VMSAVE) for the L2 guest. As a result, the L2 guest would be allowed to read/write physical pages of the host, resulting in a crash of the entire system, leak of sensitive data or potential guest-to-host escape.

Project: linux

void recalc_intercepts(struct vcpu_svm *svm)
{
	struct vmcb_control_area *c, *h, *g;
	unsigned int i;

	vmcb_mark_dirty(svm->vmcb, VMCB_INTERCEPTS);

	if (!is_guest_mode(&svm->vcpu))
		return;

	c = &svm->vmcb->control;
	h = &svm->vmcb01.ptr->control;
	g = &svm->nested.ctl;

	for (i = 0; i < MAX_INTERCEPT; i++)
		c->intercepts[i] = h->intercepts[i];

	if (g->int_ctl & V_INTR_MASKING_MASK) {
		/* We only want the cr8 intercept bits of L1 */
		vmcb_clr_intercept(c, INTERCEPT_CR8_READ);
		vmcb_clr_intercept(c, INTERCEPT_CR8_WRITE);

		/*
		 * Once running L2 with HF_VINTR_MASK, EFLAGS.IF does not
		 * affect any interrupt we may want to inject; therefore,
		 * interrupt window vmexits are irrelevant to L0.
		 */
		vmcb_clr_intercept(c, INTERCEPT_VINTR);
	}

	/* We don't want to see VMMCALLs from a nested guest */
	vmcb_clr_intercept(c, INTERCEPT_VMMCALL);

	for (i = 0; i < MAX_INTERCEPT; i++)
		c->intercepts[i] |= g->intercepts[i];

	/* If SMI is not intercepted, ignore guest SMI intercept as well  */
	if (!intercept_smi)
		vmcb_clr_intercept(c, INTERCEPT_SMI);
}

Project: linux

void recalc_intercepts(struct vcpu_svm *svm)
{
	struct vmcb_control_area *c, *h, *g;
	unsigned int i;

	vmcb_mark_dirty(svm->vmcb, VMCB_INTERCEPTS);

	if (!is_guest_mode(&svm->vcpu))
		return;

	c = &svm->vmcb->control;
	h = &svm->vmcb01.ptr->control;
	g = &svm->nested.ctl;

	for (i = 0; i < MAX_INTERCEPT; i++)
		c->intercepts[i] = h->intercepts[i];

	if (g->int_ctl & V_INTR_MASKING_MASK) {
		/* We only want the cr8 intercept bits of L1 */
		vmcb_clr_intercept(c, INTERCEPT_CR8_READ);
		vmcb_clr_intercept(c, INTERCEPT_CR8_WRITE);

		/*
		 * Once running L2 with HF_VINTR_MASK, EFLAGS.IF does not
		 * affect any interrupt we may want to inject; therefore,
		 * interrupt window vmexits are irrelevant to L0.
		 */
		vmcb_clr_intercept(c, INTERCEPT_VINTR);
	}

	/* We don't want to see VMMCALLs from a nested guest */
	vmcb_clr_intercept(c, INTERCEPT_VMMCALL);

	for (i = 0; i < MAX_INTERCEPT; i++)
		c->intercepts[i] |= g->intercepts[i];

	/* If SMI is not intercepted, ignore guest SMI intercept as well  */
	if (!intercept_smi)
		vmcb_clr_intercept(c, INTERCEPT_SMI);

	vmcb_set_intercept(c, INTERCEPT_VMLOAD);
	vmcb_set_intercept(c, INTERCEPT_VMSAVE);
}