# CVE-2026-64419: mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show()

> A bug in the Linux kernel's shrinker debugfs handler causes a sleeping function call within an RCU critical section, leading to a kernel panic and denial of service.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 25 Jul 2026
- **Updated:** 17 Aug 2026
- **Also known as:** BELL-CVE-2026-64419, DEBIAN-CVE-2026-64419, GHSA-h85j-5g9p-ppj5, ROOT-OS-UBUNTU-2204-CVE-2026-64419, UBUNTU-CVE-2026-64419

## What is CVE-2026-64419?

CVE-2026-64419 is a medium-severity denial of service vulnerability in Linux kernel, published on 25 Jul 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:

mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show()

Reading the debugfs "count" file of a memcg-aware shrinker can sleep
inside an RCU read-side critical section:

  BUG: sleeping function called from invalid context at kernel/cgroup/rstat.c:421
  RCU nest depth: 1, expected: 0
   css_rstat_flush
   mem_cgroup_flush_stats
   zswap_shrinker_count
   shrinker_debugfs_count_show

shrinker_debugfs_count_show() invokes the ->count_objects() callback under
rcu_read_lock().  The zswap callback flushes memcg stats via
css_rstat_flush(), which may sleep, so it must not run under RCU.

The RCU lock is not needed here.  mem_cgroup_iter() takes RCU internally
and returns a memcg holding a css reference (dropped on the next iteration
or by mem_cgroup_iter_break()), so the memcg stays alive without it.  The
shrinker is kept alive by the open debugfs file: shrinker_free() removes
the debugfs entries via debugfs_remove_recursive(), which waits for
in-flight readers to drain, before call_rcu(..., shrinker_free_rcu_cb). 
The sibling "scan" handler already invokes the sleeping ->scan_objects()
callback with no RCU section.

Drop the rcu_read_lock()/rcu_read_unlock().

## Which versions are affected by CVE-2026-64419?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-64419?

Apply the vendor's patch.

## Is CVE-2026-64419 being exploited?

There is no known exploitation of CVE-2026-64419 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64419 and fix it if it is affected.

What it is: CVE-2026-64419 is a medium-severity denial of service vulnerability in Linux kernel, published on 25 Jul 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-2026-64419 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-64419/ (Markdown: https://insights.sbomify.com/CVE-2026-64419/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64419
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64419
- OSV: https://osv.dev/vulnerability/CVE-2026-64419

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64419/
Summaries are generated and may contain mistakes; the official description above is the record.