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

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>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 8e853a5fc867b742321a78076d5bd81764abbc55..f66a0e59cb6a9766875277df826b9e2630d34647 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -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.

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 3b996c1fdaabc06a9f2686dacb4aa06ebd108cc8..3c65834687421a5ddd5350f1251c283be6b4a9dd 100644 (file)
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -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