# CVE-2025-40008: kmsan: fix out-of-bounds access to shadow memory

> A bug in KMSAN shadow memory handling causes a kernel crash when processing unaligned buffers near guard pages, requiring a patched kernel.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Not stated
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 20 Oct 2025
- **Updated:** 23 May 2026
- **Also known as:** BELL-CVE-2025-40008, DEBIAN-CVE-2025-40008, GHSA-jv2g-f3mc-8f2c, ROOT-OS-DEBIAN-12-CVE-2025-40008, ROOT-OS-DEBIAN-13-CVE-2025-40008, ROOT-OS-UBUNTU-2404-CVE-2025-40008, UBUNTU-CVE-2025-40008

## What is CVE-2025-40008?

CVE-2025-40008 is a memory corruption vulnerability in Linux, published on 20 Oct 2025. 

### Official description

In the Linux kernel, the following vulnerability has been resolved:

kmsan: fix out-of-bounds access to shadow memory

Running sha224_kunit on a KMSAN-enabled kernel results in a crash in
kmsan_internal_set_shadow_origin():

    BUG: unable to handle page fault for address: ffffbc3840291000
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page
    PGD 1810067 P4D 1810067 PUD 192d067 PMD 3c17067 PTE 0
    Oops: 0000 [#1] SMP NOPTI
    CPU: 0 UID: 0 PID: 81 Comm: kunit_try_catch Tainted: G                 N  6.17.0-rc3 #10 PREEMPT(voluntary)
    Tainted: [N]=TEST
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
    RIP: 0010:kmsan_internal_set_shadow_origin+0x91/0x100
    [...]
    Call Trace:
    <TASK>
    __msan_memset+0xee/0x1a0
    sha224_final+0x9e/0x350
    test_hash_buffer_overruns+0x46f/0x5f0
    ? kmsan_get_shadow_origin_ptr+0x46/0xa0
    ? __pfx_test_hash_buffer_overruns+0x10/0x10
    kunit_try_run_case+0x198/0xa00

This occurs when memset() is called on a buffer that is not 4-byte aligned
and extends to the end of a guard page, i.e.  the next page is unmapped.

The bug is that the loop at the end of kmsan_internal_set_shadow_origin()
accesses the wrong shadow memory bytes when the address is not 4-byte
aligned.  Since each 4 bytes are associated with an origin, it rounds the
address and size so that it can access all the origins that contain the
buffer.  However, when it checks the corresponding shadow bytes for a
particular origin, it incorrectly uses the original unrounded shadow
address.  This results in reads from shadow memory beyond the end of the
buffer's shadow memory, which crashes when that memory is not mapped.

To fix this, correctly align the shadow address before accessing the 4
shadow bytes corresponding to each origin.

## Which versions are affected by CVE-2025-40008?


## How do I fix CVE-2025-40008?

Apply the vendor's patch.

## Is CVE-2025-40008 being exploited?

There is no known exploitation of CVE-2025-40008 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40008 and fix it if it is affected.

What it is: CVE-2025-40008 is a memory corruption vulnerability in Linux, published on 20 Oct 2025. 
Exploitation: There is no known exploitation of CVE-2025-40008 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-2025-40008/ (Markdown: https://insights.sbomify.com/CVE-2025-40008/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40008
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40008
- OSV: https://osv.dev/vulnerability/CVE-2025-40008

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40008/
Summaries are generated and may contain mistakes; the official description above is the record.