CVE-2026-89928 KVM: x86/mmu: Consume the locked rmap value in the lockless rmap walk
A race condition in KVM's lockless rmap walk allows a use-after-free, potentially enabling local attackers to execute code or escalate privileges.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89928?
CVE-2026-89928 is a high-severity use after free vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86/mmu: Consume the locked rmap value in the lockless rmap walk
__kvm_rmap_lock() deliberately elides the rmap lock when it observes an empty rmap. In that case kvm_rmap_lock_readonly() also re-enables preemption and returns zero, so the caller holds neither the rmap lock nor a preemption reference. The elision documents the invariant it relies on:
* Elide the lock if the rmap is empty, as lockless walkers (read-only * mode) don't need to (and can't) walk an empty rmap, nor can they add * entries to the rmap. I.e. the only paths that process empty rmaps * do so while holding mmu_lock for write, and are mutually exclusive.
kvm_rmap_age_gfn_range() ignores the returned value and unconditionally enters for_each_rmap_spte_lockless(). The iterator started with rmap_get_first(), which re-reads rmap_head->val rather than using the value returned by the lock. If a writer populates the rmap between the lock's read and the iterator's re-read, the aging path walks the newly installed rmap without holding its lock.
For a KVM_RMAP_MANY rmap this leaves the walker following a pte_list_desc chain that it never locked. A writer holding mmu_lock for write may free that chain (e.g. kvm_zap_all_rmap_sptes() on the recycle path, or any rmap zap) via kmem_cache_free() while the walk is in progress, giving a slab use-after-free. Nothing serialises the two: the aging path runs without mmu_lock when CONFIG_KVM_MMU_LOCKLESS_AGING=y, and the rmap lock that would otherwise exclude the writer was elided. Because the empty path re-enables preemption, the interval between the two reads can span an arbitrary scheduling delay.
Fix the class of bug by having the lockless walk consume the value returned by the lock instead of re-reading the rmap. Split rmap_get_first() into __rmap_get_first(), which starts an iterator from an already-read rmap value, and make for_each_rmap_spte_lockless() take that value and call __rmap_get_first() directly. kvm_rmap_age_gfn_range() passes the value returned by kvm_rmap_lock_readonly(): when the lock was elided the value is zero, __rmap_get_first() returns NULL, and the walk is skipped. No lockless walker re-reads the rmap, so the lock-elision invariant cannot be violated, and no lock()-without-paired-unlock() path is added to the aging code.
Which versions are affected by CVE-2026-89928?
How do I fix CVE-2026-89928?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-89928 being exploited?
There is no known exploitation of CVE-2026-89928 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-89928, DEBIAN-CVE-2026-89928, GHSA-f7hc-2wxr-4jfg, UBUNTU-CVE-2026-89928.