# 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.

- **Severity:** High 8.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H`)
- **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:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-89928, DEBIAN-CVE-2026-89928, GHSA-f7hc-2wxr-4jfg, UBUNTU-CVE-2026-89928

## 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.

## Is CVE-2026-89928 being exploited?

There is no known exploitation of CVE-2026-89928 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89928 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-89928 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89928/ (Markdown: https://insights.sbomify.com/CVE-2026-89928/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89928
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89928
- OSV: https://osv.dev/vulnerability/CVE-2026-89928

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89928/
Summaries are generated and may contain mistakes; the official description above is the record.