FAILED: patch "[PATCH] KVM: x86/mmu: Fix write-protection of PTs mapped by the TDP" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 7c8a4742c4abe205ec9daf416c9d42fd6b406e8e Mon Sep 17 00:00:00 2001
From: David Matlack <[email protected]>
Date: Thu, 13 Jan 2022 23:30:17 +0000
Subject: [PATCH] KVM: x86/mmu: Fix write-protection of PTs mapped by the TDP
MMU

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 it fails to check if the SPTE is write-protected for page table protection, i.e. fails to check that MMU-writable is '0'. If the SPTE was write-protected for dirty logging but not page table protection, the SPTE could locklessly be made writable, and vCPUs could still be running with writable mappings 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. Technically, checking only MMU-writable would suffice; a SPTE cannot be writable without MMU-writable being set. But check both to be paranoid and because it arguably yields more readable code.

Fixes 46044f72c382 ("kvm: x86/mmu: Support write protection for nesting in tdp MMU")

Cc [email protected]

Signed-off-by David Matlack <[email protected]>

Message-Id: <[email protected]>

Reviewed-by Sean Christopherson <[email protected]>

Signed-off-by Paolo Bonzini <[email protected]>

arch/x86/kvm/mmu/tdp_mmu.c +3 -3

index 7b1bc816b7c3..bc9e3553fba2 100644
@@ -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;
        }

Quoted 6 lines · On Sun, Jan 23, 2022 at 5:36 AM <[email protected]> wrote:

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

I'll take a look and send a backport to 5.10.

Quoted 20 lines

index 7b1bc816b7c3..bc9e3553fba2 100644

arch/x86/kvm/mmu/tdp_mmu.c +3 -3

@@ -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;
         }