# CVE-2026-90136: platform/x86/amd/hsmp: Reject negative power cap writes in hwmon

> A local attacker with root privileges can write negative values to the AMD HSMP power cap, causing an integer overflow that sends invalid multi-gigawatt limits to the SMU.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **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-2026-90136, DEBIAN-CVE-2026-90136, GHSA-547r-p462-qv3c, UBUNTU-CVE-2026-90136

## What is CVE-2026-90136?

CVE-2026-90136 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:

platform/x86/amd/hsmp: Reject negative power cap writes in hwmon

hsmp_hwmon_write() takes the user-supplied hwmon value as a signed long
and assigns "val / MICROWATT_PER_MILLIWATT" to msg.args[0], which is a
__u32.  MICROWATT_PER_MILLIWATT is an unsigned long, so a negative write
to power1_cap (e.g. "echo -1 > power1_cap") is first converted to a huge
unsigned value by the division and then stored into the u32 argument.

As a result a nonsensical, multi-gigawatt socket power limit is sent to
the SMU via HSMP_SET_SOCKET_POWER_LIMIT instead of the write being
rejected.

Reject negative values with -EINVAL before the conversion.

Tested with HSMP enabled:

  CAP=$(dirname $(grep -l amd_hsmp_hwmon \
        /sys/class/hwmon/hwmon*/name | head -1))/power1_cap

  # negative write
  echo -1000000 > $CAP ; echo "ret=$?"
  # valid positive write must still work
  echo 400000000 > $CAP ; echo "ret=$?"

Before:
  # echo -1000000 > $CAP ; echo "ret=$?"
  ret=0                             <- accepted; bogus limit sent to SMU
  # echo 400000000 > $CAP ; echo "ret=$?"
  ret=0

After:
  # echo -1000000 > $CAP ; echo "ret=$?"
  bash: echo: write error: Invalid argument
  ret=1                             <- rejected with -EINVAL
  # echo 400000000 > $CAP ; echo "ret=$?"
  ret=0                             <- valid write still works

## Which versions are affected by CVE-2026-90136?


## How do I fix CVE-2026-90136?

Apply the vendor's patch.

## Is CVE-2026-90136 being exploited?

There is no known exploitation of CVE-2026-90136 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90136 and fix it if it is affected.

What it is: CVE-2026-90136 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-2026-90136 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-2026-90136/ (Markdown: https://insights.sbomify.com/CVE-2026-90136/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90136
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90136
- OSV: https://osv.dev/vulnerability/CVE-2026-90136

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90136/
Summaries are generated and may contain mistakes; the official description above is the record.