Re: [RFC PATCH] KVM: x86: Fix APIC page invalidation race

[Paolo Bonzini]

On 06/06/20 06:26, Eiichi Tsukata wrote:
> Commit b1394e745b94 ("KVM: x86: fix APIC page invalidation") tried to
> fix inappropriate APIC page invalidation by re-introducing arch specific
> kvm_arch_mmu_notifier_invalidate_range() and calling it from
> kvm_mmu_notifier_invalidate_range_start. But threre could be the
> following race because VMCS APIC address cache can be updated
> *before* it is unmapped.
> 
> Race:
>   (Invalidator) kvm_mmu_notifier_invalidate_range_start()
>   (Invalidator) kvm_make_all_cpus_request(kvm, KVM_REQ_APIC_PAGE_RELOAD)
>   (KVM VCPU) vcpu_enter_guest()
>   (KVM VCPU) kvm_vcpu_reload_apic_access_page()
>   (Invalidator) actually unmap page
> 
> Symptom:
>   The above race can make Guest OS see already freed page and Guest OS
> will see broken APIC register values.

This is not exactly the issue.  The values in the APIC-access page do
not really matter, the problem is that the host physical address values
won't match between the page tables and the APIC-access page address.
Then the processor will not trap APIC accesses, and will instead show
the raw contents of the APIC-access page (zeroes), and cause the crash
as you mention below.

Still, the race explains the symptoms and the patch matches this text in
include/linux/mmu_notifier.h:

	 * If the subsystem
         * can't guarantee that no additional references are taken to
         * the pages in the range, it has to implement the
         * invalidate_range() notifier to remove any references taken
         * after invalidate_range_start().

where the "additional reference" is in the VMCS: because we have to
account for kvm_vcpu_reload_apic_access_page running between
invalidate_range_start() and invalidate_range_end(), we need to
implement invalidate_range().

The patch seems good, but I'd like Andrea Arcangeli to take a look as
well so I've CCed him.

Thank you very much!

Paolo

> Especially, Windows OS checks
> LAPIC modification so it can cause BSOD crash with BugCheck
> CRITICAL_STRUCTURE_CORRUPTION (109). These symptoms are the same as we
> previously saw in https://bugzilla.kernel.org/show_bug.cgi?id=197951 and
> we are currently seeing in
> https://bugzilla.redhat.com/show_bug.cgi?id=1751017.
> 
> To prevent Guest OS from accessing already freed page, this patch calls
> kvm_arch_mmu_notifier_invalidate_range() from
> kvm_mmu_notifier_invalidate_range() instead of ..._range_start().