On Tue, Jan 18, 2022, Vitaly Kuznetsov wrote:
> +/* Check whether the supplied CPUID data is equal to what is already set for the vCPU. */
> +static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
> + int nent)
> +{
> + struct kvm_cpuid_entry2 *orig;
> + int i;
> +
> + if (nent != vcpu->arch.cpuid_nent)
> + return -EINVAL;
> +
> + for (i = 0; i < nent; i++) {
> + orig = &vcpu->arch.cpuid_entries[i];
> + if (e2[i].function != orig->function ||
> + e2[i].index != orig->index ||
> + e2[i].eax != orig->eax || e2[i].ebx != orig->ebx ||
> + e2[i].ecx != orig->ecx || e2[i].edx != orig->edx)
> + return -EINVAL;
This needs to check .flags for the above check on .index to be meaningful, and at
that point, can't we be even more agressive and just do?
if (memcmp(e2, vcpu->arch.cpuid_entries, nent * sizeof(e2)))
return -EINVAL;
return 0;
> + }
> +
> + return 0;
> +}
> +
> static void kvm_update_kvm_cpuid_base(struct kvm_vcpu *vcpu)
> {
> u32 function;
> @@ -313,6 +335,20 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
>
> __kvm_update_cpuid_runtime(vcpu, e2, nent);
> + /*
> + * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
> + * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
> + * tracked in kvm_mmu_page_role. As a result, KVM may miss guest page
> + * faults due to reusing SPs/SPTEs. In practice no sane VMM mucks with
> + * the core vCPU model on the fly. It would've been better to forbid any
> + * KVM_SET_CPUID{,2} calls after KVM_RUN altogether but unfortunately
> + * some VMMs (e.g. QEMU) reuse vCPU fds for CPU hotplug/unplug and do
> + * KVM_SET_CPUID{,2} again. To support this legacy behavior, check
> + * whether the supplied CPUID data is equal to what's already set.
This is misleading/wrong. KVM_RUN isn't the only problematic ioctl(), it's just
the one that we decided to use to detect that userspace is being stupid. And
forbidding KVM_SET_CPUID after KVM_RUN (or even all problematic ioctls()) wouldn't
solve problem as providing different CPUID configurations for vCPUs in a VM will
also cause the MMU to fall on its face.
> + if (vcpu->arch.last_vmentry_cpu != -1)
> + return kvm_cpuid_check_equal(vcpu, e2, nent);
And technically, checking last_vmentry_cpu doesn't forbid changing CPUID after
KVM_RUN, it forbids changing CPUID after successfully entering the guest (or
emulating instructions on VMX).
I realize I'm being very pedantic, as a well-intended userspace is obviously not
going to change CPUID after -EINTR or whatever. But I do want to highlight that
this approach is by no means bulletproof, and that what is/isn't allowed with
respect to guest CPUID isn't necessarily associated with what is/isn't "safe".
In other words, this check doesn't guarantee that userspace can't misuse KVM_SET_CPUID,
and on the flip side it disallows using KVM_SET_CPUID in ways that are perfectly ok
(if userspace is careful and deliberate).
