Re: [PATCH v2 4/4] KVM: x86/mmu: Fix a *very* theoretical race in kvm_mmu_track_write()

[Paolo Bonzini]

On 2/3/24 01:23, Sean Christopherson wrote:
> Add full memory barriers in kvm_mmu_track_write() and account_shadowed()
> to plug a (very, very theoretical) race where kvm_mmu_track_write() could
> miss a 0->1 transition of indirect_shadow_pages and fail to zap relevant,
> *stale* SPTEs.

Ok, so we have

emulator_write_phys
  overwrite PTE
  kvm_page_track_write
    kvm_mmu_track_write
      // memory barrier missing here
      if (indirect_shadow_pages)
        zap();

and on the other side

  FNAME(page_fault)
    FNAME(fetch)
      kvm_mmu_get_child_sp
        kvm_mmu_get_shadow_page
          __kvm_mmu_get_shadow_page
            kvm_mmu_alloc_shadow_page
              account_shadowed
                indirect shadow pages++
                // memory barrier missing here
      if (FNAME(gpte_changed)) // reads PTE
        goto out

If you can weave something like this in the commit message the sequence 
would be a bit clearer.

> In practice, this bug is likely benign as both the 0=>1 transition and
> reordering of this scope are extremely rare occurrences.

I wouldn't call it benign, it's more that it's unobservable in practice 
but the race is real.  However...
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 3c193b096b45..86b85060534d 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -830,6 +830,14 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
>   	struct kvm_memory_slot *slot;
>   	gfn_t gfn;
>   
> +	/*
> +	 * Ensure indirect_shadow_pages is elevated prior to re-reading guest
> +	 * child PTEs in FNAME(gpte_changed), i.e. guarantee either in-flight
> +	 * emulated writes are visible before re-reading guest PTEs, or that
> +	 * an emulated write will see the elevated count and acquire mmu_lock
> +	 * to update SPTEs.  Pairs with the smp_mb() in kvm_mmu_track_write().
> +	 */
> +	smp_mb();

.. this memory barrier needs to be after the increment (the desired 
ordering is store-before-read).

Paolo

>   	kvm->arch.indirect_shadow_pages++;
>   	gfn = sp->gfn;
>   	slots = kvm_memslots_for_spte_role(kvm, sp->role);
> @@ -5747,10 +5755,15 @@ void kvm_mmu_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
>   	bool flush = false;
>   
>   	/*
> -	 * If we don't have indirect shadow pages, it means no page is
> -	 * write-protected, so we can exit simply.
> +	 * When emulating guest writes, ensure the written value is visible to
> +	 * any task that is handling page faults before checking whether or not
> +	 * KVM is shadowing a guest PTE.  This ensures either KVM will create
> +	 * the correct SPTE in the page fault handler, or this task will see
> +	 * a non-zero indirect_shadow_pages.  Pairs with the smp_mb() in
> +	 * account_shadowed().
>   	 */
> -	if (!READ_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
> +	smp_mb();
> +	if (!vcpu->kvm->arch.indirect_shadow_pages)
>   		return;
>   
>   	write_lock(&vcpu->kvm->mmu_lock);