# CVE-2026-63951: zram: fix use-after-free in zram_writeback_endio

> A race condition in the Linux kernel zram writeback handler causes a use-after-free, leading to a system crash.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/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:** 19 Jul 2026
- **Updated:** 5 Aug 2026
- **Also known as:** DEBIAN-CVE-2026-63951, GHSA-jmwm-428x-w863, UBUNTU-CVE-2026-63951

## What is CVE-2026-63951?

CVE-2026-63951 is a high-severity use after free vulnerability in Linux, published on 19 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:

zram: fix use-after-free in zram_writeback_endio

A crash was observed in zram_writeback_endio due to a NULL pointer
dereference in wake_up.  The root cause is a race condition between the
bio completion handler (zram_writeback_endio) and the writeback task.

In zram_writeback_endio, wake_up() is called on &wb_ctl->done_wait after
releasing wb_ctl->done_lock.  This creates a race window where the
writeback task can see num_inflight become 0, return, and free wb_ctl
before zram_writeback_endio calls wake_up().

CPU 0 (zram_writeback_endio)     CPU 1 (writeback_store)
============================     ============================
                                 zram_writeback_slots
                                   zram_submit_wb_request
                                   zram_submit_wb_request
                                   wait_event(wb_ctl->done_wait)
spin_lock(&wb_ctl->done_lock);
list_add(&req->entry, &wb_ctl->done_reqs);
spin_unlock(&wb_ctl->done_lock);
wake_up(&wb_ctl->done_wait);
                                   zram_complete_done_reqs
spin_lock(&wb_ctl->done_lock);
list_add(&req->entry, &wb_ctl->done_reqs);
spin_unlock(&wb_ctl->done_lock);
                                   while (num_inflight) > 0)
                                     spin_lock(&wb_ctl->done_lock);
                                     list_del(&req->entry);
                                     spin_unlock(&wb_ctl->done_lock);
                                     // num_inflight becomes 0
                                     atomic_dec(num_inflight);

                                 // Leave zram_writeback_slots
                                 // Free wb_ctl
                                 release_wb_ctl(wb_ctl);
// UAF crash!
wake_up(&wb_ctl->done_wait);

This patch fixes this race by using RCU.  By protecting wb_ctl with
rcu_read_lock() in zram_writeback_endio and using kfree_rcu() to free it,
we ensure that wb_ctl remains valid during the execution of
zram_writeback_endio.

## Which versions are affected by CVE-2026-63951?


## How do I fix CVE-2026-63951?

Apply the vendor's patch.

## Is CVE-2026-63951 being exploited?

There is no known exploitation of CVE-2026-63951 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63951 and fix it if it is affected.

What it is: CVE-2026-63951 is a high-severity use after free vulnerability in Linux, published on 19 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-2026-63951 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-2026-63951/ (Markdown: https://insights.sbomify.com/CVE-2026-63951/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63951
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63951
- OSV: https://osv.dev/vulnerability/CVE-2026-63951

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63951/
Summaries are generated and may contain mistakes; the official description above is the record.