ntfs: bound the free-cluster bitmap scan to the volume

Overview

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

ntfs: bound the free-cluster bitmap scan to the volume

vol->lcn_empty_bits_per_page is sized from vol->nr_clusters at mount, but ntfs_cluster_alloc() bounds its scan of that array by the size of $Bitmap. Those are independent on-disk quantities and the mount-time check only rejects a $Bitmap that is too small, so an image whose $Bitmap covers more clusters than the volume has lets the scan index past the array. A run whose LCN lies in that gap takes the allocator straight there, since the caller passes the file's own last LCN as its locality hint. KASAN reports a slab out-of-bounds read when a file on such a volume is extended.

Clamp the scan to what that array covers, mirroring the max_index calculation the mount-time scan already uses, and reject a decoded LCN at or beyond nr_clusters in the mapping pairs decoder. Conforming volumes are unaffected.

Affected

• Linux >= 11ccc9107dc460de28af90fac1f42404d9802735 < dc15a9f5307d5372aef3b64bf3ed7802f530c471
• Linux >= 11ccc9107dc460de28af90fac1f42404d9802735 < 19cac7902a8ab748e15f98ddaafcf5f8882be21d
• Linux 7.1

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Moderate · updated 2026-09-16 · vex