On Wed, Oct 13, 2021 at 9:02 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Wed, Sep 29, 2021, David Stevens wrote: > > From: David Stevens <stevensd@xxxxxxxxxxxx> > > > > Remove two warnings that require ref counts for pages to be non-zero, as > > mapped pfns from follow_pfn may not have an initialized ref count. > > > > Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx> > > --- > > arch/x86/kvm/mmu/mmu.c | 7 ------- > > virt/kvm/kvm_main.c | 2 +- > > 2 files changed, 1 insertion(+), 8 deletions(-) > > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > > index 5a1adcc9cfbc..3b469df63bcf 100644 > > --- a/arch/x86/kvm/mmu/mmu.c > > +++ b/arch/x86/kvm/mmu/mmu.c > > @@ -617,13 +617,6 @@ static int mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep) > > > > pfn = spte_to_pfn(old_spte); > > > > - /* > > - * KVM does not hold the refcount of the page used by > > - * kvm mmu, before reclaiming the page, we should > > - * unmap it from mmu first. > > - */ > > - WARN_ON(!kvm_is_reserved_pfn(pfn) && !page_count(pfn_to_page(pfn))); > > Have you actually observed false positives with this WARN? I would expect anything > without a struct page to get filtered out by !kvm_is_reserved_pfn(pfn). Those are the type of pfns that were responsible for CVE-2021-22543 [1]. One specific example is that amdgpu uses ttm_pool, which makes higher order, non-compound allocation. Without the head/tail metadata, only the first base page in such an allocation has non-zero page_count. [1] https://github.com/google/security-research/security/advisories/GHSA-7wq5-phmq-m584 > If you have observed false positives, I would strongly prefer we find a way to > keep the page_count() sanity check, it has proven very helpful in the past in > finding/debugging bugs during MMU development. When we see a refcount of zero, I think we can look up spte->(gfn, slot)->hva->vma and determine whether or not the zero refcount is okay, based on vm_flags. That's kind of heavy for a debug check, although at least we'd only pay the cost for unusual mappings. But it still might make sense to switch to a MMU_WARN_ON, in that case. Or we could just ignore the cost, since at least from a superficial reading and some basic tests, tdp_mmu doesn't seem to execute this code path. Thoughts? I'd lean towards MMU_WARN_ON, but I'd like to know what the maintainers' preferences are before sending an updated patch series. -David > > > - > > if (is_accessed_spte(old_spte)) > > kvm_set_pfn_accessed(pfn); > >
