CVE-2026-72478: fs/ntfs3: add bounds check to run_get_highest_vcn()

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

fs/ntfs3: add bounds check to rungethighestvcn()

rungethighestvcn() parses a packed NTFS mapping-pairs buffer without any length bound, relying solely on a 0x00 terminator to stop. A crafted $LogFile UpdateMappingPairs record whose embedded attribute contains mapping-pairs runs without a terminator causes the function to read past the slab allocation, triggering a KASAN slab-out-of-bounds read on mount.

The sibling function rununpack() received an analogous bounds-check in commit b62567bca474 ("ntfs3: add buffer boundary checks to rununpack()"), but rungethighestvcn() was missed.

Take a runbufsize parameter and reject any run header whose payload would extend past the buffer end, mirroring the pattern used by rununpack(). The caller in fslog.c passes the remaining attribute bytes after the mapping-pairs offset.

KASAN report (on mainline v7.1 merge window HEAD):

BUG: KASAN: slab-out-of-bounds in rungethighestvcn+0x3c0/0x410 Read of size 1 at addr ffff88800e2d5400 by task mount/72 Call Trace: rungethighestvcn+0x3c0/0x410 doaction.isra.0+0x3ba8/0x7b50 logreplay+0x9ddd/0x10200 ntfsloadlogandreplay+0x4ad/0x610 ntfsfillsuper+0x214a/0x4540