Linux Kernel FUSE fuse_direct_write_iter ki_pos information disclosure

Summaryinfo

A vulnerability, which was classified as problematic, has been found in Linux Kernel. The impacted element is the function fuse_direct_write_iter of the component FUSE. The manipulation of the argument ki_pos leads to information disclosure. This vulnerability is traded as CVE-2026-90096. It is possible to initiate the attack remotely. There is no exploit available. Applying a patch is the recommended action to fix this issue.

Detailsinfo

A vulnerability was found in Linux Kernel (affected version unknown) and classified as problematic. This issue affects the function fuse_direct_write_iter of the component FUSE. The manipulation of the argument ki_pos with an unknown input leads to a information disclosure vulnerability. Using CWE to declare the problem leads to CWE-200. The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information. Impacted is confidentiality. The summary by CVE is:

In the Linux kernel, the following vulnerability has been resolved: fuse: invalidate the correct range after O_APPEND direct write fuse_direct_write_iter() captures pos before generic_write_checks(), which moves ki_pos to EOF for O_APPEND writes: fuse_direct_write_iter() { pos = iocb->ki_pos; /* 0 (user-supplied) */ generic_write_checks(); /* ki_pos -> EOF */ fuse_direct_io(); /* writes at EOF, correct */ invalidate(pos, pos + res); /* [0, res) -- wrong */ } The post-write invalidation targets a stale range instead of the actual written range at EOF. This can cause data inconsistency when the file size is not page-aligned. The tail page straddling EOF has a valid portion before EOF that concurrent readers can fault back in during the DIO write window: Tail page (file size X not page-aligned): page_start X (EOF) page_end |--- valid data ----|-- stale --| CPU0 (O_APPEND DIO writer) CPU1 (buffered reader) -------------------------- ---------------------- invalidate [X, X+len) tail page evicted FUSE_WRITE in flight ... read [page_start, X) tail page re-faulted [X, page_end) = stale FUSE_WRITE completes i_size = X + len invalidate [0, len) <- WRONG tail page still cached read [X, X+len) hits stale tail page returns old data Fix by reading pos back from iocb->ki_pos after generic_write_checks(), as generic_file_direct_write() does. Also fix a typo in the comment ("may have" -> "may have competed").

The advisory is shared at git.kernel.org. The identification of this vulnerability is CVE-2026-90096 since 09/11/2026. The exploitation is known to be easy. The attack may be initiated remotely. It demands that the victim is doing some kind of user interaction. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 09/17/2026). MITRE ATT&CK project uses the attack technique T1592 for this issue.

Applying the patch 833963069adf86dcbdffd4e7d7b3171f95070b77/26d7e1f5c407b5859122b5cd47d7ebbf4b4c1cd2 is able to eliminate this problem.

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Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

License

• open-source

Website

• Vendor: https://www.kernel.org/

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