CVE-2026-98135

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

ntfs: reject invalid sectors_per_cluster in the boot sector

is_boot_sector_ntfs() checks the boot sector's sectors_per_cluster field with a range test that rejects 0x81..0xf3 but accepts 0 and other non-power-of-two counts. A zero value reaches parse_ntfs_boot_sector():

sectors_per_cluster_bits = ffs(sectors_per_cluster) - 1; ... vol->cluster_size = vol->sector_size << sectors_per_cluster_bits;

ffs(0) is 0, so sectors_per_cluster_bits becomes (unsigned)-1 and the shift is undefined:

UBSAN: shift-out-of-bounds in fs/ntfs/super.c:673:39 shift exponent 4294967295 is too large for 32-bit type 'int'

This change rejects any non-power-of-two value, since it feeds the aforementioned shift via ffs() - 1, which only yields the correct shift for a power of two.