[PATCH 1/4] KVM: x86/mmu: Check kvm_mmu_page_ad_need_write_protect() when clearing TDP MMU dirty bits

[David Matlack <dmatlack@google.com>]

Check kvm_mmu_page_ad_need_write_protect() when deciding whether to
write-protect or clear D-bits on TDP MMU SPTEs.

TDP MMU SPTEs must be write-protected when the TDP MMU is being used to
run an L2 (i.e. L1 has disabled EPT) and PML is enabled. KVM always
disables the PML hardware when running L2, so failing to write-protect
TDP MMU SPTEs will cause writes made by L2 to not be reflected in the
dirty log.

Reported-by: syzbot+900d58a45dcaab9e4821@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=900d58a45dcaab9e4821
Fixes: 5982a5392663 ("KVM: x86/mmu: Use kvm_ad_enabled() to determine if TDP MMU SPTEs need wrprot")
Cc: stable@vger.kernel.org
Cc: Vipin Sharma <vipinsh@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 6ae19b4ee5b1..c3c1a8f430ef 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1498,6 +1498,16 @@ void kvm_tdp_mmu_try_split_huge_pages(struct kvm *kvm,
 	}
 }
 
+static bool tdp_mmu_need_write_protect(struct kvm_mmu_page *sp)
+{
+	/*
+	 * All TDP MMU shadow pages share the same role as their root, aside
+	 * from level, so it is valid to key off any shadow page to determine if
+	 * write protection is needed for an entire tree.
+	 */
+	return kvm_mmu_page_ad_need_write_protect(sp) || !kvm_ad_enabled();
+}
+
 /*
  * Clear the dirty status of all the SPTEs mapping GFNs in the memslot. If
  * AD bits are enabled, this will involve clearing the dirty bit on each SPTE.
@@ -1508,7 +1518,8 @@ void kvm_tdp_mmu_try_split_huge_pages(struct kvm *kvm,
 static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
 			   gfn_t start, gfn_t end)
 {
-	u64 dbit = kvm_ad_enabled() ? shadow_dirty_mask : PT_WRITABLE_MASK;
+	const u64 dbit = tdp_mmu_need_write_protect(root)
+		? PT_WRITABLE_MASK : shadow_dirty_mask;
 	struct tdp_iter iter;
 	bool spte_set = false;
 
@@ -1523,7 +1534,7 @@ static bool clear_dirty_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
 		if (tdp_mmu_iter_cond_resched(kvm, &iter, false, true))
 			continue;
 
-		KVM_MMU_WARN_ON(kvm_ad_enabled() &&
+		KVM_MMU_WARN_ON(dbit == shadow_dirty_mask &&
 				spte_ad_need_write_protect(iter.old_spte));
 
 		if (!(iter.old_spte & dbit))
@@ -1570,8 +1581,8 @@ bool kvm_tdp_mmu_clear_dirty_slot(struct kvm *kvm,
 static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
 				  gfn_t gfn, unsigned long mask, bool wrprot)
 {
-	u64 dbit = (wrprot || !kvm_ad_enabled()) ? PT_WRITABLE_MASK :
-						   shadow_dirty_mask;
+	const u64 dbit = (wrprot || tdp_mmu_need_write_protect(root))
+		? PT_WRITABLE_MASK : shadow_dirty_mask;
 	struct tdp_iter iter;
 
 	lockdep_assert_held_write(&kvm->mmu_lock);
@@ -1583,7 +1594,7 @@ static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
 		if (!mask)
 			break;
 
-		KVM_MMU_WARN_ON(kvm_ad_enabled() &&
+		KVM_MMU_WARN_ON(dbit == shadow_dirty_mask &&
 				spte_ad_need_write_protect(iter.old_spte));
 
 		if (iter.level > PG_LEVEL_4K ||
-- 
2.44.0.291.gc1ea87d7ee-goog