[PATCH v2 1/2] KVM: x86/mmu: Check write tracking in all address spaces

[Jinu Kim]

kvm_gfn_is_write_tracked() checks only the supplied memslot, but page
tracking is per-address-space and shadow pages are shared across all
address spaces.  With SMM, a GFN can therefore be write-tracked in one
address space and appear untracked through the other.

Check the supplied slot first, then the slot for the other address space.
This ensures all callers honor write tracking regardless of the active
address space.  In particular, it prevents mmu_try_to_unsync_pages() from
marking an upper-level shadow page unsync and eventually triggering the
BUG in pte_list_remove().

Fixes: 699023e23965 ("KVM: x86: add SMM to the MMU role, support SMRAM address space")
Assisted-by: Codex:GPT-5
Signed-off-by: Jinu Kim <kimjw04271234@xxxxxxxxx>
---
 arch/x86/kvm/mmu/page_track.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
index 1b17b12393a8c..ff93591be1adb 100644
--- a/arch/x86/kvm/mmu/page_track.c
+++ b/arch/x86/kvm/mmu/page_track.c
@@ -133,10 +133,22 @@ void __kvm_write_track_remove_gfn(struct kvm *kvm,
 /*
  * check if the corresponding access on the specified guest page is tracked.
  */
+static bool __kvm_gfn_is_write_tracked(const struct kvm_memory_slot *slot,
+				       gfn_t gfn)
+{
+	int index;
+
+	if (!slot)
+		return false;
+
+	index = gfn_to_index(gfn, slot->base_gfn, PG_LEVEL_4K);
+	return !!READ_ONCE(slot->arch.gfn_write_track[index]);
+}
+
 bool kvm_gfn_is_write_tracked(struct kvm *kvm,
 			      const struct kvm_memory_slot *slot, gfn_t gfn)
 {
-	int index;
+	const struct kvm_memory_slot *other_slot;
 
 	if (!slot)
 		return false;
@@ -144,8 +156,16 @@ bool kvm_gfn_is_write_tracked(struct kvm *kvm,
 	if (!kvm_page_track_write_tracking_enabled(kvm))
 		return false;
 
-	index = gfn_to_index(gfn, slot->base_gfn, PG_LEVEL_4K);
-	return !!READ_ONCE(slot->arch.gfn_write_track[index]);
+	BUILD_BUG_ON(KVM_MAX_NR_ADDRESS_SPACES > 2);
+
+	if (__kvm_gfn_is_write_tracked(slot, gfn))
+		return true;
+
+	if (kvm_arch_nr_memslot_as_ids(kvm) == 1)
+		return false;
+
+	other_slot = __gfn_to_memslot(__kvm_memslots(kvm, slot->as_id ^ 1), gfn);
+	return __kvm_gfn_is_write_tracked(other_slot, gfn);
 }
 
 #ifdef CONFIG_KVM_EXTERNAL_WRITE_TRACKING

base-commit: 6bc96b971766fbbbbdd9fb2642cedacaf02da957
-- 
2.43.0