[PATCH v13 05/85] KVM: x86/mmu: Don't overwrite shadow-present MMU SPTEs when prefaulting

Sean Christopherson
2024-10-10T18:37:45Z

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>
---
arch/x86/kvm/mmu/mmu.c | 3 +++
arch/x86/kvm/mmu/tdp_mmu.c | 3 +++
2 files changed, 6 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index a9a23e058555..a8c64069aa89 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2919,6 +2919,9 @@ 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 3b996c1fdaab..3c6583468742 100644
--- 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