CVE-2026-90096 in Linux
Summary
by MITRE • 09/17/2026
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").
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel FUSE subsystem contains a logic error within the fuse_direct_write_iter function that leads to data inconsistency during append-mode direct I/O operations when file sizes are not page-aligned. The vulnerability stems from an incorrect sequence of variable capture and cache invalidation relative to position updates performed by generic write checks. Specifically, the implementation captures the initial user-supplied file offset into a local pos variable before invoking generic_write_checks. This check function is responsible for adjusting the kernel I/O control block's ki_pos field to reflect the end-of-file location when an O_APPEND flag is present in the open flags. Consequently, while the actual direct IO operation correctly writes data at the new end-of-file position, the subsequent cache invalidation routine operates on the stale pos variable which still holds the original user-supplied offset rather than the updated EOF-based offset.
This discrepancy results in the kernel attempting to invalidate a memory range that does not correspond to the physical location of the written data. In scenarios where the file size is not aligned with the system page boundary, this error has tangible consequences for concurrent readers accessing the same file through buffered I/O paths. The tail page straddling the end-of-file contains valid data up to the previous EOF and stale or uninitialized data beyond that point. During a direct IO write window, if a reader faults in the tail page before the invalidation completes, it may cache portions of the page that include both valid pre-EOF data and potentially stale post-EOF regions. When the FUSE write operation eventually completes and triggers an incorrect invalidation on the old range, the kernel fails to properly evict or update the actual pages containing the newly written data at EOF.
The operational impact manifests as a race condition where concurrent buffered readers can retrieve outdated or corrupted data from the tail page of the file. This occurs because the stale cache entries are not invalidated in the correct memory region corresponding to the new write location, allowing old data to persist in the page cache and be served to subsequent read operations. Such behavior violates data integrity guarantees expected by applications relying on consistent file contents across different I/O modes. The flaw is classified under CWE-362 which covers concurrent execution errors leading to race conditions that compromise system security or stability. From an ATT&CK perspective, this aligns with techniques involving memory corruption and exploitation of timing differences in kernel operations, although it primarily affects data integrity rather than direct privilege escalation unless leveraged for specific denial-of-service scenarios.
The resolution involves correcting the control flow within fuse_direct_write_iter to ensure that the position variable used for cache invalidation accurately reflects the final write location after all positional adjustments have been applied by generic helpers. By reading ki_pos back from the I/O control block immediately following the call to generic_write_checks, the kernel ensures that invalidate_range calls target the correct memory pages associated with the actual data written at EOF. This aligns the FUSE implementation behavior with standard direct write patterns found elsewhere in the VFS layer such as generic_file_direct_write. Additionally, minor documentation updates were applied to clarify comments regarding completion states of asynchronous operations. System administrators should apply kernel patches that include this fix to prevent potential data corruption issues in environments utilizing FUSE mounts with append-mode direct I/O and concurrent buffered reads on non-page-aligned files.