CVE-2025-21896 fuse: revert back to __readahead_folio() for readahead
A use-after-free vulnerability in the Linux kernel's FUSE filesystem allows attackers to potentially execute code or escalate privileges via splice pipe responses.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-21896?
CVE-2025-21896 is a high-severity use after free vulnerability in Linux kernel, published on 1 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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.
Weakness: CWE-416
Which versions are affected by CVE-2025-21896?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21896?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2025-21896 being exploited?
There is no known exploitation of CVE-2025-21896 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2025-21896, GHSA-4r9v-r5j3-85m7, UBUNTU-CVE-2025-21896.