KVM: x86/mmu: Skip the "try unsync" path iff the old SPTE was a leaf SPTE

Apply make_spte()'s optimization to skip trying to unsync shadow pages if
and only if the old SPTE was a leaf SPTE, as non-leaf SPTEs in direct MMUs
are always writable, i.e. could trigger a false positive and incorrectly
lead to KVM creating a SPTE without write-protecting or marking shadow
pages unsync.

This bug only affects the TDP MMU, as the shadow MMU only overwrites a
shadow-present SPTE when synchronizing SPTEs (and only 4KiB SPTEs can be
unsync).  Specifically, mmu_set_spte() drops any non-leaf SPTEs *before*
calling make_spte(), whereas the TDP MMU can do a direct replacement of a
page table with the leaf SPTE.

Opportunistically update the comment to explain why skipping the unsync
stuff is safe, as opposed to simply saying "it's someone else's problem".

Cc: stable@vger.kernel.org
Tested-by: Alex Bennée <alex.bennee@linaro.org>
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: <20241010182427.1434605-5-seanjc@google.com>

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index 8f7eb3ad88fcb97469ee9e3c5d5a02440b18ea77..5521608077ec093d29da442407dd12745430ec08 100644 (file)
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -226,12 +226,20 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
                spte |= PT_WRITABLE_MASK | shadow_mmu_writable_mask;
 
                /*
-                * Optimization: for pte sync, if spte was writable the hash
-                * lookup is unnecessary (and expensive). Write protection
-                * is responsibility of kvm_mmu_get_page / kvm_mmu_sync_roots.
-                * Same reasoning can be applied to dirty page accounting.
+                * When overwriting an existing leaf SPTE, and the old SPTE was
+                * writable, skip trying to unsync shadow pages as any relevant
+                * shadow pages must already be unsync, i.e. the hash lookup is
+                * unnecessary (and expensive).
+                *
+                * The same reasoning applies to dirty page/folio accounting;
+                * KVM will mark the folio dirty using the old SPTE, thus
+                * there's no need to immediately mark the new SPTE as dirty.
+                *
+                * Note, both cases rely on KVM not changing PFNs without first
+                * zapping the old SPTE, which is guaranteed by both the shadow
+                * MMU and the TDP MMU.
                 */
-               if (is_writable_pte(old_spte))
+               if (is_last_spte(old_spte, level) && is_writable_pte(old_spte))
                        goto out;
 
                /*