# CVE-2025-40124: sparc: fix accurate exception reporting in copy_{from_to}_user for UltraSPARC III

> A flaw in SPARC copy_from_user exception handling causes incorrect byte counts, leading to kernel BUG_ON crashes and denial of service.

- **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:** 12 Nov 2025
- **Updated:** 5 Aug 2026
- **Also known as:** BELL-CVE-2025-40124, DEBIAN-CVE-2025-40124, GHSA-v3c5-2xf8-7jwx, ROOT-OS-DEBIAN-11-CVE-2025-40124, ROOT-OS-DEBIAN-13-CVE-2025-40124, ROOT-OS-UBUNTU-2404-CVE-2025-40124, UBUNTU-CVE-2025-40124

## What is CVE-2025-40124?

CVE-2025-40124 is a high-severity memory corruption vulnerability in Linux, published on 12 Nov 2025. 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:

sparc: fix accurate exception reporting in copy_{from_to}_user for UltraSPARC III

Anthony Yznaga tracked down that a BUG_ON in ext4 code with large folios
enabled resulted from copy_from_user() returning impossibly large values
greater than the size to be copied. This lead to __copy_from_iter()
returning impossible values instead of the actual number of bytes it was
able to copy.

The BUG_ON has been reported in
https://lore.kernel.org/r/b14f55642207e63e907965e209f6323a0df6dcee.camel@physik.fu-berlin.de

The referenced commit introduced exception handlers on user-space memory
references in copy_from_user and copy_to_user. These handlers return from
the respective function and calculate the remaining bytes left to copy
using the current register contents. The exception handlers expect that
%o2 has already been masked during the bulk copy loop, but the masking was
performed after that loop. This will fix the return value of copy_from_user
and copy_to_user in the faulting case. The behaviour of memcpy stays
unchanged.

## Which versions are affected by CVE-2025-40124?


## How do I fix CVE-2025-40124?

Apply the vendor's patch.

## Is CVE-2025-40124 being exploited?

There is no known exploitation of CVE-2025-40124 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40124 and fix it if it is affected.

What it is: CVE-2025-40124 is a high-severity memory corruption vulnerability in Linux, published on 12 Nov 2025. 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-2025-40124 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-2025-40124/ (Markdown: https://insights.sbomify.com/CVE-2025-40124/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40124
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40124
- OSV: https://osv.dev/vulnerability/CVE-2025-40124

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40124/
Summaries are generated and may contain mistakes; the official description above is the record.