Re: [PATCH] KVM: x86/tdp_mmu: Trigger the callback only when an interesting change

Yan Zhao
2024-09-26T07:55:47Z

On Thu, Sep 12, 2024 at 05:07:57PM -0700, Sean Christopherson wrote:
> On Thu, Sep 12, 2024, Isaku Yamahata wrote:
> > KVM MMU behavior
> > ================
> > The leaf SPTE state machine is coded in make_spte(). Consider AD bits and
> > the present bits for simplicity. The two functionalities and AD bits
> > support are related in this context. unsync (manipulate D bit and W bit,
> > and handle write protect fault) and access tracking (manipulate A bit and
> > present bit, and hand handle page fault). (We don't consider dirty page
> > tracking for now as it's future work of TDX KVM.)
> >
> > * If AD bit is enabled:
> > D bit state change for dirty page tracking
> > On the first EPT violation without prefetch,
> > - D bits are set.
> > - Make SPTE writable as TDX supports only RXW (or if write fault).
> > (TDX KVM doesn't support write protection at this state. It's future work.)
> >
> > On the second EPT violation.
> > - clear D bits if write fault.
>
> Heh, I was literally just writing changelogs for patches to address this (I told
> Sagi I would do it "this week"; that was four weeks ago).
>
> This is a bug in make_spte(). Replacing a W=1,D=1 SPTE with a W=1,D=0 SPTE is
> nonsensical. And I'm pretty sure it's an outright but for the TDP MMU (see below).
>
> Right now, the fixes for make_spte() are sitting toward the end of the massive
> kvm_follow_pfn() rework (80+ patches and counting), but despite the size, I am
> fairly confident that series can land in 6.13 (lots and lots of small patches).
>
> ---
> Author: Sean Christopherson <seanjc@google.com>
> AuthorDate: Thu Sep 12 16:23:21 2024 -0700
> Commit: Sean Christopherson <seanjc@google.com>
> CommitDate: Thu Sep 12 16:35:06 2024 -0700
>
> KVM: x86/mmu: Flush TLBs if resolving a TDP MMU fault clears W or D bits
>
> Do a remote TLB flush if installing a leaf SPTE overwrites an existing
> leaf SPTE (with the same target pfn) and clears the Writable bit or the
> Dirty bit. KVM isn't _supposed_ to clear Writable or Dirty bits in such
> a scenario, but make_spte() has a flaw where it will fail to set the Dirty
> if the existing SPTE is writable.
>
> E.g. if two vCPUs race to handle faults, the KVM will install a W=1,D=1
> SPTE for the first vCPU, and then overwrite it with a W=1,D=0 SPTE for the
> second vCPU. If the first vCPU (or another vCPU) accesses memory using
> the W=1,D=1 SPTE, i.e. creates a writable, dirty TLB entry, and that is
> the only SPTE that is dirty at the time of the next relevant clearing of
> the dirty logs, then clear_dirty_gfn_range() will not modify any SPTEs
> because it sees the D=0 SPTE, and thus will complete the clearing of the
> dirty logs without performing a TLB flush.
But it looks that kvm_flush_remote_tlbs_memslot() will always be invoked no
matter clear_dirty_gfn_range() finds a D bit or not.

kvm_mmu_slot_apply_flags
|kvm_mmu_slot_leaf_clear_dirty
| |kvm_tdp_mmu_clear_dirty_slot
| |clear_dirty_gfn_range
|kvm_flush_remote_tlbs_memslot

> Opportunistically harden the TDP MMU against clearing the Writable bit as
> well, both to prevent similar bugs for write-protection, but also so that
> the logic can eventually be deduplicated into spte.c (mmu_spte_update() in
> the shadow MMU has similar logic).
>
> Fix the bug in the TDP MMU's page fault handler even though make_spte() is
> clearly doing odd things, e.g. it marks the page dirty in its slot but
> doesn't set the Dirty bit. Precisely because replacing a leaf SPTE with
> another leaf SPTE is so rare, the cost of hardening KVM against such bugs
> is negligible. The make_spte() will be addressed in a future commit.
>
> Fixes: bb18842e2111 ("kvm: x86/mmu: Add TDP MMU PF handler")
> Cc: David Matlack <dmatlack@google.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 3b996c1fdaab..7c6d1c610f0e 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1038,7 +1038,9 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
> else if (tdp_mmu_set_spte_atomic(vcpu->kvm, iter, new_spte))
> return RET_PF_RETRY;
> else if (is_shadow_present_pte(iter->old_spte) &&
> - !is_last_spte(iter->old_spte, iter->level))
> + (!is_last_spte(iter->old_spte, iter->level) ||
> + (is_mmu_writable_spte(old_spte) && !is_writable_pte(new_spte)) ||
> + (is_dirty_spte(old_spte) && !is_dirty_spte(new_spte))))
Should we also check is_accessed_spte() as what's done in mmu_spte_update()?

It is possible for make_spte() to make the second spte !is_dirty_spte(), e.g.
the second one is caused by a KVM_PRE_FAULT_MEMORY ioctl.

> kvm_flush_remote_tlbs_gfn(vcpu->kvm, iter->gfn, iter->level);
>
> /*
> ---
>
> > arch/x86/kvm/mmu/spte.h | 6 ++++++
> > arch/x86/kvm/mmu/tdp_mmu.c | 28 +++++++++++++++++++++++++---
> > 2 files changed, 31 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> > index a72f0e3bde17..1726f8ec5a50 100644
> > --- a/arch/x86/kvm/mmu/spte.h
> > +++ b/arch/x86/kvm/mmu/spte.h
> > @@ -214,6 +214,12 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
> > */
> > #define FROZEN_SPTE (SHADOW_NONPRESENT_VALUE | 0x5a0ULL)
> >
> > +#define EXTERNAL_SPTE_IGNORE_CHANGE_MASK \
> > + (shadow_acc_track_mask | \
> > + (SHADOW_ACC_TRACK_SAVED_BITS_MASK << \
> > + SHADOW_ACC_TRACK_SAVED_BITS_SHIFT) | \
>
> Just make TDX require A/D bits, there's no reason to care about access tracking.
If KVM_PRE_FAULT_MEMORY is allowed for TDX and if
cpu_has_vmx_ept_ad_bits() is false in TDX's hardware (not sure if it's possible),
access tracking bit is possbile to be changed, as in below scenario:

1. KVM_PRE_FAULT_MEMORY ioctl calls kvm_arch_vcpu_pre_fault_memory() to map
a GFN, and make_spte() will call mark_spte_for_access_track() to
remove shadow_acc_track_mask (i.e. RWX bits) and move R+X left to
SHADOW_ACC_TRACK_SAVED_BITS_SHIFT.
2. If a concurrent page fault occurs on the same GFN on another vCPU, then
make_spte() in that vCPU will not see prefetch and the new_spte is
with RWX bits and with no bits set in SHADOW_ACC_TRACK_SAVED_MASK.

>
> > + shadow_dirty_mask | shadow_accessed_mask)
> > +
> > /* Removed SPTEs must not be misconstrued as shadow present PTEs. */
> > static_assert(!(FROZEN_SPTE & SPTE_MMU_PRESENT_MASK));
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 019b43723d90..cfb82ede8982 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -503,8 +503,6 @@ static int __must_check set_external_spte_present(struct kvm *kvm, tdp_ptep_t sp
> > kvm_pfn_t new_pfn = spte_to_pfn(new_spte);
> > int ret = 0;
> >
> > - KVM_BUG_ON(was_present, kvm);
> > -
> > lockdep_assert_held(&kvm->mmu_lock);
> > /*
> > * We need to lock out other updates to the SPTE until the external
> > @@ -519,10 +517,34 @@ static int __must_check set_external_spte_present(struct kvm *kvm, tdp_ptep_t sp
> > * external page table, or leaf.
> > */
> > if (is_leaf) {
> > - ret = static_call(kvm_x86_set_external_spte)(kvm, gfn, level, new_pfn);
> > + /*
> > + * SPTE is state machine with software available bits used.
> > + * Check if the change is interesting to the backend.
> > + */
> > + if (!was_present)
> > + ret = static_call(kvm_x86_set_external_spte)(kvm, gfn, level, new_pfn);
> > + else {
> > + /*
> > + * The external PTEs don't need updates for some bits,
> > + * but if others are changed, bug the VM.
> > + */
> > + if (KVM_BUG_ON(~EXTERNAL_SPTE_IGNORE_CHANGE_MASK &
>
> There's no reason to bug the VM. WARN, yes (maybe), but not bug the VM. And I
> think this should be short-circuited somewhere further up the chain, i.e. not
> just for TDX.
>
> One idea would be to WARN and skip setting the SPTE in tdp_mmu_map_handle_target_level().
> I.e. WARN and ignore 1=>0 transitions for Writable and Dirty bits, and then drop
> the TLB flush (assuming the above patch lands).
>
> > + (old_spte ^ new_spte), kvm)) {
>
> Curly braces are unnecessary.
>
> > + ret = -EIO;
> > + }
> > + }
> > +
> > + /*
> > + * The backend shouldn't return an error except EAGAIN.
> > + * It's hard to debug without those info.
> > + */
> > + if (ret && ret != EAGAIN)
> > + pr_debug("gfn 0x%llx old_spte 0x%llx new_spte 0x%llx level %d\n",
> > + gfn, old_spte, new_spte, level);
>
> Please no. Not in upstream. Yeah, for development it's fine, but sprinkling
> printks all over the MMU eventually just results in stale printks, e.g. see all
> of the pgprintk crud that got ripped out a while back.
>
> > } else {
> > void *external_spt = get_external_spt(gfn, new_spte, level);
> >
> > + KVM_BUG_ON(was_present, kvm);
> > KVM_BUG_ON(!external_spt, kvm);
> > ret = static_call(kvm_x86_link_external_spt)(kvm, gfn, level, external_spt);
> > }
> >
> > base-commit: d2c7662a6ea1c325a9ae878b3f1a265264bcd18b
> > --
> > 2.45.2
> >
>