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KVM: x86/mmu: Don't overwrite shadow-present MMU SPTEs when prefaulting

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Treat attempts to prefetch/prefault MMU SPTEs as spurious if there's an
existing shadow-present SPTE, as overwriting a SPTE that may have been
create by a "real" fault is at best confusing, and at worst potentially
harmful.  E.g. mmu_try_to_unsync_pages() doesn't unsync when prefetching,
which creates a scenario where KVM could try to replace a Writable SPTE
with a !Writable SPTE, as sp->unsync is checked prior to acquiring
mmu_unsync_pages_lock.

Note, this applies to three of the four flavors of "prefetch" in KVM:

  - KVM_PRE_FAULT_MEMORY
  - Async #PF (host or PV)
  - Prefetching

The fourth flavor, SPTE synchronization, i.e. FNAME(sync_spte), _only_
overwrites shadow-present SPTEs when calling make_spte().  But SPTE
synchronization specifically uses mmu_spte_update(), and so naturally
avoids the @prefetch check in mmu_set_spte().

Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-6-seanjc@google.com>
This commit is contained in:
Sean Christopherson
2024-10-10 11:23:07 -07:00
committed by Paolo Bonzini
parent 2867eb782c
commit 6385d01eec
2 changed files with 6 additions and 0 deletions
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3
arch/x86/kvm/mmu/mmu.c
View File

@@ -2926,6 +2926,9 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
}
if (is_shadow_present_pte(*sptep)) {
if (prefetch)
return RET_PF_SPURIOUS;
/*
* If we overwrite a PTE page pointer with a 2MB PMD, unlink
* the parent of the now unreachable PTE.

3
arch/x86/kvm/mmu/tdp_mmu.c
View File

@@ -1026,6 +1026,9 @@ static int tdp_mmu_map_handle_target_level(struct kvm_vcpu *vcpu,
if (WARN_ON_ONCE(sp->role.level != fault->goal_level))
return RET_PF_RETRY;
if (fault->prefetch && is_shadow_present_pte(iter->old_spte))
return RET_PF_SPURIOUS;
if (unlikely(!fault->slot))
new_spte = make_mmio_spte(vcpu, iter->gfn, ACC_ALL);
else