# CVE-2026-90400: md: recheck spare changes before starting sync

> A race condition in the Linux kernel md driver allows a local attacker to trigger a NULL pointer dereference, causing a system crash.

- **Severity:** Not scored
- **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:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-90400, DEBIAN-CVE-2026-90400, GHSA-vwgq-hpcr-5x9r, UBUNTU-CVE-2026-90400

## What is CVE-2026-90400?

CVE-2026-90400 is a vulnerability in Linux, published on 17 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:

md: recheck spare changes before starting sync

remove_spares() and remove_and_add_spares() modify the array's rdev
configuration. These operations are only safe after the array has been
suspended.

md_start_sync() checks whether spare configuration changes are needed
before taking reconfig_mutex. However, the rdev state can change before
the mutex is acquired, so the initial check can become stale. In that
case, md_choose_sync_action() may remove or replace rdevs while normal
I/O is still accessing them.

The race can occur as follows:

raid10d          Worker                      Normal IO
____________     _______________________     ______________________

                                             raid10_write_request()
                                             wait_blocked_dev()
set Blocked
set Faulty
                                             Skip Faulty rdev
                                             rrdev->nr_pending++
                                             .repl_bio = bio
                 removeable_rdev = false     .
                 array not suspended         .
lock mddev                                   goto err_handle
                 lock mddev (wait)
                 .
update sb        .
clear Blocked    .
                 .
unlock mddev     .
                 lock mddev (acquires)
                 remove_spares()
                 removeable_rdev = true

                 raid10_remove_disk()
                 rdev = replacement
                 replacement = NULL
                                             rdev_dec_pending(NULL)
                 unlock mddev                (NULL)->nr_pending--

In this case, rdev_dec_pending() is called with a NULL pointer,
resulting in a NULL pointer dereference when attempting to decrement
nr_pending.

Fix this by suspending the array when spare configuration changes are
needed, including for non-read-write arrays, and checking again after
taking reconfig_mutex. If the array was not already suspended and a
change is now needed, release the mutex, suspend the array, and
reacquire the mutex before continuing.

## Which versions are affected by CVE-2026-90400?


## How do I fix CVE-2026-90400?

Apply the vendor's patch.

## Is CVE-2026-90400 being exploited?

There is no known exploitation of CVE-2026-90400 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90400 and fix it if it is affected.

What it is: CVE-2026-90400 is a vulnerability in Linux, published on 17 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-2026-90400 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-2026-90400/ (Markdown: https://insights.sbomify.com/CVE-2026-90400/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90400
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90400
- OSV: https://osv.dev/vulnerability/CVE-2026-90400

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90400/
Summaries are generated and may contain mistakes; the official description above is the record.