CVE-2025-21896: linux kernel Use-After-Free — Fix

Description

In the Linux kernel, the following vulnerability has been resolved: fuse: revert back to __readahead_folio() for readahead In commit 3eab9d7bc2f4 ("fuse: convert readahead to use folios"), the logic was converted to using the new folio readahead code, which drops the reference on the folio once it is locked, using an inferred reference on the folio. Previously we held a reference on the folio for the entire duration of the readpages call. This is fine, however for the case for splice pipe responses where we will remove the old folio and splice in the new folio (see fuse_try_move_page()), we assume that there is a reference held on the folio for ap->folios, which is no longer the case. To fix this, revert back to __readahead_folio() which allows us to hold the reference on the folio for the duration of readpages until either we drop the reference ourselves in fuse_readpages_end() or the reference is dropped after it's replaced in the page cache in the splice case. This will fix the UAF bug that was reported.

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CVSS v3.1 Score

7.8

HIGH

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Weakness Type (CWE)

CWE-416 Use After Free

Affected Products

Vendor	Product	Versions
linux	linux_kernel	6.13 — 6.13.6
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All

References

Advisories & Patches

https://git.kernel.org/stable/c/0c67c37e1710b2a8f61c8a02db95a51fe577e2c1 https://git.kernel.org/stable/c/60db11f1b7fba4a66b117ea998d965818784a98d

Frequently Asked Questions

What is CVE-2025-21896? +

In the Linux kernel, the following vulnerability has been resolved: fuse: revert back to __readahead_folio() for readahead In commit 3eab9d7bc2f4 ("fuse: convert readahead to use folios"), the logic was converted to using the new folio readahead code, which drops the reference on the folio once it is locked, using an inferred reference on the folio. Previously we held a reference on the folio for the entire duration of the readpages call. This is fine, however for the case for splice pipe responses where we will remove the old folio and splice in the new folio (see fuse_try_move_page()), we assume that there is a reference held on the folio for ap->folios, which is no longer the case. To fix this, revert back to __readahead_folio() which allows us to hold the reference on the folio for the duration of readpages until either we drop the reference ourselves in fuse_readpages_end() or the reference is dropped after it's replaced in the page cache in the splice case. This will fix the UAF bug that was reported. It has a CVSS v3.1 base score of 7.8 (HIGH).

How severe is CVE-2025-21896? +

CVE-2025-21896 has a CVSS v3.1 score of 7.8 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

What products are affected by CVE-2025-21896? +

CVE-2025-21896 affects products from linux, specifically: linux_kernel. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-21896? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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