Commit message: KVM: SVM: load control fields from VMCB12 before checking them

Avoid races between check and use of the nested VMCB controls.  This
for example ensures that the VMRUN intercept is always reflected to the
nested hypervisor, instead of being processed by the host.  Without this
patch, it is possible to end up with svm->nested.hsave pointing to
the MSR permission bitmap for nested guests.

This bug is CVE-2021-29657.

Reported-by: Felix Wilhelm <fwilhelm@google.com>
Cc: stable@vger.kernel.org
Fixes: 2fcf4876ada ("KVM: nSVM: implement on demand allocation of the nested state")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
CVE: CVE-2021-29657
CWE: CWE-416
Vulnerability description: arch/x86/kvm/svm/nested.c in the Linux kernel before 5.11.12 has a use-after-free in which an AMD KVM guest can bypass access control on host OS MSRs when there are nested guests, aka CID-a58d9166a756. This occurs because of a TOCTOU race condition associated with a VMCB12 double fetch in nested_svm_vmrun.

Project: linux
File: nested.c

int nested_svm_vmrun(struct vcpu_svm *svm)
{
	int ret;
	struct vmcb *vmcb12;
	struct vmcb *hsave = svm->nested.hsave;
	struct vmcb *vmcb = svm->vmcb;
	struct kvm_host_map map;
	u64 vmcb12_gpa;

	if (is_smm(&svm->vcpu)) {
		kvm_queue_exception(&svm->vcpu, UD_VECTOR);
		return 1;
	}

	vmcb12_gpa = svm->vmcb->save.rax;
	ret = kvm_vcpu_map(&svm->vcpu, gpa_to_gfn(vmcb12_gpa), &map);
	if (ret == -EINVAL) {
		kvm_inject_gp(&svm->vcpu, 0);
		return 1;
	} else if (ret) {
		return kvm_skip_emulated_instruction(&svm->vcpu);
	}

	ret = kvm_skip_emulated_instruction(&svm->vcpu);

	vmcb12 = map.hva;

	if (WARN_ON_ONCE(!svm->nested.initialized))
		return -EINVAL;

	if (!nested_vmcb_checks(svm, vmcb12)) {
		vmcb12->control.exit_code    = SVM_EXIT_ERR;
		vmcb12->control.exit_code_hi = 0;
		vmcb12->control.exit_info_1  = 0;
		vmcb12->control.exit_info_2  = 0;
		goto out;
	}

	trace_kvm_nested_vmrun(svm->vmcb->save.rip, vmcb12_gpa,
			       vmcb12->save.rip,
			       vmcb12->control.int_ctl,
			       vmcb12->control.event_inj,
			       vmcb12->control.nested_ctl);

	trace_kvm_nested_intercepts(vmcb12->control.intercepts[INTERCEPT_CR] & 0xffff,
				    vmcb12->control.intercepts[INTERCEPT_CR] >> 16,
				    vmcb12->control.intercepts[INTERCEPT_EXCEPTION],
				    vmcb12->control.intercepts[INTERCEPT_WORD3],
				    vmcb12->control.intercepts[INTERCEPT_WORD4],
				    vmcb12->control.intercepts[INTERCEPT_WORD5]);

	/* Clear internal status */
	kvm_clear_exception_queue(&svm->vcpu);
	kvm_clear_interrupt_queue(&svm->vcpu);

	/*
	 * Save the old vmcb, so we don't need to pick what we save, but can
	 * restore everything when a VMEXIT occurs
	 */
	hsave->save.es     = vmcb->save.es;
	hsave->save.cs     = vmcb->save.cs;
	hsave->save.ss     = vmcb->save.ss;
	hsave->save.ds     = vmcb->save.ds;
	hsave->save.gdtr   = vmcb->save.gdtr;
	hsave->save.idtr   = vmcb->save.idtr;
	hsave->save.efer   = svm->vcpu.arch.efer;
	hsave->save.cr0    = kvm_read_cr0(&svm->vcpu);
	hsave->save.cr4    = svm->vcpu.arch.cr4;
	hsave->save.rflags = kvm_get_rflags(&svm->vcpu);
	hsave->save.rip    = kvm_rip_read(&svm->vcpu);
	hsave->save.rsp    = vmcb->save.rsp;
	hsave->save.rax    = vmcb->save.rax;
	if (npt_enabled)
		hsave->save.cr3    = vmcb->save.cr3;
	else
		hsave->save.cr3    = kvm_read_cr3(&svm->vcpu);

	copy_vmcb_control_area(&hsave->control, &vmcb->control);

	svm->nested.nested_run_pending = 1;

	if (enter_svm_guest_mode(svm, vmcb12_gpa, vmcb12))
		goto out_exit_err;

	if (nested_svm_vmrun_msrpm(svm))
		goto out;

out_exit_err:
	svm->nested.nested_run_pending = 0;

	svm->vmcb->control.exit_code    = SVM_EXIT_ERR;
	svm->vmcb->control.exit_code_hi = 0;
	svm->vmcb->control.exit_info_1  = 0;
	svm->vmcb->control.exit_info_2  = 0;

	nested_svm_vmexit(svm);

out:
	kvm_vcpu_unmap(&svm->vcpu, &map, true);

	return ret;
}