Linux EUVD-2026-81872

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

md/raid5: round bitmap stripes with sector division

raid5_bitmap_sector_map() aligns the array range to full RAID5 stripe widths before converting it to component sectors. That width is chunk_sectors multiplied by the number of data disks, and it is not always a power of two.

Reproduce with a 4-disk RAID5, 1024-sector chunks, and three data disks. The full-stripe width is 3072 sectors. For a one-sector write at array sector 3072, correct rounding gives array range [3072, 6144), which maps to component range [1024, 2048). The old round_down()/round_up() logic instead gives [1024, 4096), which maps to [0, 1024).

Use sector_div() based arithmetic so the rounded range is aligned to the actual RAID5 stripe width.

The deterministic mapper test now reports the fixed component range as [1024, 2048), while the old mask-based range was [0, 1024).