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.

This vulnerability is rated Moderate severity because exploitation requires an authenticated local user with the ability to interact with FUSE filesystems, limiting the blast radius compared to remote flaws. An attacker must trigger specific readahead page-splice operations to induce memory corruption or a denial of service. Furthermore, the vulnerable folio readahead logic was introduced in recent upstream kernels and is absent from standard enterprise deployments such as RHEL 8 and 9.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.