CVE-2025-21896

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

fuse: revert back to _readaheadfolio() 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 fusetrymove_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 _readaheadfolio() which allows us to hold the reference on the folio for the duration of readpages until either we drop the reference ourselves in fusereadpagesend() 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.