On Sat, Jul 23, 2022 at 01:23:21AM +0000, Sean Christopherson wrote: > Account and track NX huge pages for nonpaging MMUs so that a future > enhancement to precisely check if shadow page cannot be replaced by a NX > huge page doesn't get false positives. Without correct tracking, KVM can > get stuck in a loop if an instruction is fetching and writing data on the > same huge page, e.g. KVM installs a small executable page on the fetch > fault, replaces it with an NX huge page on the write fault, and faults > again on the fetch. > > Alternatively, and perhaps ideally, KVM would simply not enforce the > workaround for nonpaging MMUs. The guest has no page tables to abuse > and KVM is guaranteed to switch to a different MMU on CR0.PG being > toggled so there's no security or performance concerns. However, getting > make_spte() to play nice now and in the future is unnecessarily complex. > > In the current code base, make_spte() can enforce the mitigation if TDP > is enabled or the MMU is indirect, but make_spte() may not always have a > vCPU/MMU to work with, e.g. if KVM were to support in-line huge page > promotion when disabling dirty logging. > > Without a vCPU/MMU, KVM could either pass in the correct information > and/or derive it from the shadow page, but the former is ugly and the > latter subtly non-trivial due to the possitibility of direct shadow pages > in indirect MMUs. Given that using shadow paging with an unpaged guest > is far from top priority _and_ has been subjected to the workaround since > its inception, keep it simple and just fix the accounting glitch. > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> It's odd that KVM enforced NX Huge Pages but just skipped the accounting. In retrospect, that was bound to cause some issue. Aside from the comment suggestion below, Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx> > --- > arch/x86/kvm/mmu/mmu.c | 2 +- > arch/x86/kvm/mmu/mmu_internal.h | 8 ++++++++ > arch/x86/kvm/mmu/spte.c | 11 +++++++++++ > 3 files changed, 20 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > index 1112e3a4cf3e..493cdf1c29ff 100644 > --- a/arch/x86/kvm/mmu/mmu.c > +++ b/arch/x86/kvm/mmu/mmu.c > @@ -3135,7 +3135,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) > continue; > > link_shadow_page(vcpu, it.sptep, sp); > - if (fault->is_tdp && fault->huge_page_disallowed) > + if (fault->huge_page_disallowed) > account_nx_huge_page(vcpu->kvm, sp, > fault->req_level >= it.level); > } > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h > index ff4ca54b9dda..83644a0167ab 100644 > --- a/arch/x86/kvm/mmu/mmu_internal.h > +++ b/arch/x86/kvm/mmu/mmu_internal.h > @@ -201,6 +201,14 @@ struct kvm_page_fault { > > /* Derived from mmu and global state. */ > const bool is_tdp; > + > + /* > + * Note, enforcing the NX huge page mitigation for nonpaging MMUs > + * (shadow paging, CR0.PG=0 in the guest) is completely unnecessary. > + * The guest doesn't have any page tables to abuse and is guaranteed > + * to switch to a different MMU when CR0.PG is toggled on (may not > + * always be guaranteed when KVM is using TDP). See also make_spte(). > + */ > const bool nx_huge_page_workaround_enabled; > > /* > diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c > index 7314d27d57a4..9f3e5af088a5 100644 > --- a/arch/x86/kvm/mmu/spte.c > +++ b/arch/x86/kvm/mmu/spte.c > @@ -147,6 +147,17 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp, > if (!prefetch) > spte |= spte_shadow_accessed_mask(spte); > > + /* > + * For simplicity, enforce the NX huge page mitigation even if not > + * strictly necessary. KVM could ignore if the mitigation if paging is > + * disabled in the guest, but KVM would then have to ensure a new MMU > + * is loaded (or all shadow pages zapped) when CR0.PG is toggled on, > + * and that's a net negative for performance when TDP is enabled. KVM > + * could ignore the mitigation if TDP is disabled and CR0.PG=0, as KVM > + * will always switch to a new MMU if paging is enabled in the guest, > + * but that adds complexity just to optimize a mode that is anything > + * but performance critical. > + */ I had some trouble parsing the last sentence. How about this for slightly better flow: /* * For simplicity, enforce the NX huge page mitigation even if not * strictly necessary. KVM could ignore if the mitigation if paging is * disabled in the guest, but KVM would then have to ensure a new MMU * is loaded (or all shadow pages zapped) when CR0.PG is toggled on, * and that's a net negative for performance when TDP is enabled. When * TDP is disabled, KVM will always switch to a new MMU when CR0.PG is * toggled, but that would tie make_spte() further to vCPU/MMU state * and add complexity just to optimize a mode that is anything but * performance critical. */ > if (level > PG_LEVEL_4K && (pte_access & ACC_EXEC_MASK) && > is_nx_huge_page_enabled(vcpu->kvm)) { > pte_access &= ~ACC_EXEC_MASK; > -- > 2.37.1.359.gd136c6c3e2-goog >