From: Sean Christopherson <seanjc@xxxxxxxxxx>
Assert that a page's refcount is elevated, i.e. that _something_ holds a
reference to the page, when KVM marks a page as accessed and/or dirty.
KVM typically doesn't hold a reference to pages that are mapped into the
guest, e.g. to allow page migration, compaction, swap, etc., and instead
relies on mmu_notifiers to react to changes in the primary MMU.
Incorrect handling of mmu_notifier events (or similar mechanisms) can
result in KVM keeping a mapping beyond the lifetime of the backing page,
i.e. can (and often does) result in use-after-free. Yelling if KVM marks
a freed page as accessed/dirty doesn't prevent badness as KVM usually
only does A/D updates when unmapping memory from the guest, i.e. the
assertion fires well after an underlying bug has occurred, but yelling
does help detect, triage, and debug use-after-free bugs.
Note, the assertion must use page_count(), NOT page_ref_count()! For
hugepages, the returned struct page may be a tailpage and thus not have
its own refcount.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
virt/kvm/kvm_main.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d63cf1c4f5a7..ee6090ecb1fe 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2914,6 +2914,19 @@ EXPORT_SYMBOL_GPL(kvm_vcpu_unmap);
static bool kvm_is_ad_tracked_page(struct page *page)
{
+ /*
+ * Assert that KVM isn't attempting to mark a freed page as Accessed or
+ * Dirty, i.e. that KVM's MMU doesn't have a use-after-free bug. KVM
+ * (typically) doesn't pin pages that are mapped in KVM's MMU, and
+ * instead relies on mmu_notifiers to know when a mapping needs to be
+ * zapped/invalidated. Unmapping from KVM's MMU must happen _before_
+ * KVM returns from its mmu_notifier, i.e. the page should have an
+ * elevated refcount at this point even though KVM doesn't hold a
+ * reference of its own.
+ */
+ if (WARN_ON_ONCE(!page_count(page)))
+ return false;
+
/*
* Per page-flags.h, pages tagged PG_reserved "should in general not be
* touched (e.g. set dirty) except by its owner".
--
2.42.0.283.g2d96d420d3-goog
