# CVE-2026-74738: regmap: sdw-mbq: don't call an unset readable_reg callback

> A null pointer dereference in the Linux kernel's regmap SoundWire driver allows local attackers to cause a system crash.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 Aug 2026
- **Updated:** 26 Aug 2026
- **Also known as:** BELL-CVE-2026-74738, DEBIAN-CVE-2026-74738, UBUNTU-CVE-2026-74738

## What is CVE-2026-74738?

CVE-2026-74738 is a vulnerability in Linux, published on 26 Aug 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:

regmap: sdw-mbq: don't call an unset readable_reg callback

regmap_sdw_mbq_poll_busy() decides whether to poll the Function Busy bit
by calling ctx->readable_reg(), which is a straight copy of
config->readable_reg. That callback is optional: regmap_readable() treats
a NULL ->readable_reg as "every register is readable", and drivers rely on
that. es9356 and tac5xx2-sdw both build an MBQ regmap without one.

Since commit ca1b11b36d82 ("regmap: sdw-mbq: Allow defers on undeferrable
controls") the poll runs on every -ENODATA, not only for Controls the
driver marked deferrable, so any of those devices answering
COMMAND_IGNORED takes the kernel through a NULL function pointer.

Treat a missing callback the way the rest of regmap does and poll.

## Which versions are affected by CVE-2026-74738?


## How do I fix CVE-2026-74738?

Apply the vendor's patch.

## Is CVE-2026-74738 being exploited?

There is no known exploitation of CVE-2026-74738 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74738 and fix it if it is affected.

What it is: CVE-2026-74738 is a vulnerability in Linux, published on 26 Aug 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-2026-74738 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-2026-74738/ (Markdown: https://insights.sbomify.com/CVE-2026-74738/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74738
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74738
- OSV: https://osv.dev/vulnerability/CVE-2026-74738

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74738/
Summaries are generated and may contain mistakes; the official description above is the record.