KVM: x86/mmu: Clear dirtied pages mask bit before early break

In clear_dirty_pt_masked, the loop is intended to exit early after
processing each of the GFNs with corresponding bits set in mask. This
does not work as intended if another thread has already cleared the
dirty bit or writable bit on the SPTE. In that case, the loop would
proceed to the next iteration early and the bit in mask would not be
cleared. As a result the loop could not exit early and would proceed
uselessly. Move the unsetting of the mask bit before the check for a
no-op SPTE change.

Fixes: a6a0b05da9f3 ("kvm: x86/mmu: Support dirty logging for the TDP
MMU")

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210202185734.1680553-17-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index aeb05f626b556bed7e001c2ae7e1453a6d478932..a75e92164a8b62f07c0dc8794986749eafea90cf 100644 (file)
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1007,6 +1007,8 @@ static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
                    !(mask & (1UL << (iter.gfn - gfn))))
                        continue;
 
+               mask &= ~(1UL << (iter.gfn - gfn));
+
                if (wrprot || spte_ad_need_write_protect(iter.old_spte)) {
                        if (is_writable_pte(iter.old_spte))
                                new_spte = iter.old_spte & ~PT_WRITABLE_MASK;
@@ -1020,8 +1022,6 @@ static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
                }
 
                tdp_mmu_set_spte_no_dirty_log(kvm, &iter, new_spte);
-
-               mask &= ~(1UL << (iter.gfn - gfn));
        }
 }