[PATCH v2 1/4] KVM: x86/mmu: Fix write-protection of PTs mapped by the TDP MMU

[David Matlack <dmatlack@google.com>]

When the TDP MMU is write-protection GFNs for page table protection (as
opposed to for dirty logging, or due to the HVA not being writable), it
checks if the SPTE is already write-protected and if so skips modifying
the SPTE and the TLB flush.

This behavior is incorrect because the SPTE may be write-protected for
dirty logging. This implies that the SPTE could be locklessly be made
writable on the next write access, and that vCPUs could still be running
with writable SPTEs cached in their TLB.

Fix this by only skipping setting the SPTE if the SPTE is already
write-protected *and* MMU-writable is already clear.

Fixes: 46044f72c382 ("kvm: x86/mmu: Support write protection for nesting in tdp MMU")
Cc: stable@vger.kernel.org
Signed-off-by: David Matlack <dmatlack@google.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 7b1bc816b7c3..bc9e3553fba2 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1442,12 +1442,12 @@ static bool write_protect_gfn(struct kvm *kvm, struct kvm_mmu_page *root,
 		    !is_last_spte(iter.old_spte, iter.level))
 			continue;
 
-		if (!is_writable_pte(iter.old_spte))
-			break;
-
 		new_spte = iter.old_spte &
 			~(PT_WRITABLE_MASK | shadow_mmu_writable_mask);
 
+		if (new_spte == iter.old_spte)
+			break;
+
 		tdp_mmu_set_spte(kvm, &iter, new_spte);
 		spte_set = true;
 	}

base-commit: fea31d1690945e6dd6c3e89ec5591490857bc3d4
-- 
2.34.1.703.g22d0c6ccf7-goog