---
id: CVE-2025-21664
title: |-
  In the Linux kernel, the following vulnerability has been resolved:

  dm thin: make get_first_thin use rcu-safe list first function

  The documentation in rculist.h explains the absence of list_empty_rcu()
  and cautions programmers against …
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  dm thin: make get_first_thin use rcu-safe list first function

  The documentation in rculist.h explains the absence of list_empty_rcu()
  and cautions programmers against …
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'
vendor: linux
product: linux_kernel
affected:
  - 'linux_kernel >= 3.15.1, < 5.4.290'
  - 'linux_kernel >= 5.5, < 5.10.234'
  - 'linux_kernel >= 5.11, < 5.15.177'
  - 'linux_kernel >= 5.16, < 6.1.125'
  - 'linux_kernel >= 6.2, < 6.6.72'
  - 'linux_kernel >= 6.7, < 6.12.10'
  - linux_kernel = 3.15
  - linux_kernel = 6.13
patched:
  - linux_kernel 6.12.10
published: '2025-01-21'
updated: '2026-07-30'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2025-21664'
references:
  - url: 'https://git.kernel.org/stable/c/12771050b6d059eea096993bf2001da9da9fddff'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/6b305e98de0d225ccebfb225730a9f560d28ecb0'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/802666a40c71a23542c43a3f87e3a2d0f4e8fe45'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/80f130bfad1dab93b95683fc39b87235682b8f72'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/cbd0d5ecfa390ac29c5380200147d09c381b2ac6'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/cd30a3960433ec2db94b3689752fa3c5df44d649'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/ec037fe8c0d0f6140e3d8a49c7b29cb5582160b8'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://lists.debian.org/debian-lts-announce/2025/03/msg00001.html'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://lists.debian.org/debian-lts-announce/2025/03/msg00002.html'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://cert-portal.siemens.com/productcert/html/ssa-265688.html'
    label: 0b142b55-0307-4c5a-b3c9-f314f3fb7c5e
tags:
  - nvd
epss: 0.00236
epssPercentile: 0.14831
ingestedAt: '2026-07-30T06:53:09.074Z'
---

## Overview

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

dm thin: make get_first_thin use rcu-safe list first function

The documentation in rculist.h explains the absence of list_empty_rcu()
and cautions programmers against relying on a list_empty() ->
list_first() sequence in RCU safe code.  This is because each of these
functions performs its own READ_ONCE() of the list head.  This can lead
to a situation where the list_empty() sees a valid list entry, but the
subsequent list_first() sees a different view of list head state after a
modification.

In the case of dm-thin, this author had a production box crash from a GP
fault in the process_deferred_bios path.  This function saw a valid list
head in get_first_thin() but when it subsequently dereferenced that and
turned it into a thin_c, it got the inside of the struct pool, since the
list was now empty and referring to itself.  The kernel on which this
occurred printed both a warning about a refcount_t being saturated, and
a UBSAN error for an out-of-bounds cpuid access in the queued spinlock,
prior to the fault itself.  When the resulting kdump was examined, it
was possible to see another thread patiently waiting in thin_dtr's
synchronize_rcu.

The thin_dtr call managed to pull the thin_c out of the active thins
list (and have it be the last entry in the active_thins list) at just
the wrong moment which lead to this crash.

Fortunately, the fix here is straight forward.  Switch get_first_thin()
function to use list_first_or_null_rcu() which performs just a single
READ_ONCE() and returns NULL if the list is already empty.

This was run against the devicemapper test suite's thin-provisioning
suites for delete and suspend and no regressions were observed.

## Affected

- `linux_kernel >= 3.15.1, < 5.4.290`
- `linux_kernel >= 5.5, < 5.10.234`
- `linux_kernel >= 5.11, < 5.15.177`
- `linux_kernel >= 5.16, < 6.1.125`
- `linux_kernel >= 6.2, < 6.6.72`
- `linux_kernel >= 6.7, < 6.12.10`
- `linux_kernel = 3.15`
- `linux_kernel = 6.13`

## Remediation

Upgrade past the affected range:

- `linux_kernel 6.12.10`