The validation contained an off-by-one error. The
expression '(u32)(usa_ofs + (usa_count * 2)) > size' used 'usa_count'
after it had been decremented to skip the update sequence number entry.
Consequently, the code could read out of bounds, up to two bytes past the
end of the MST-protected record.
Furthermore, as documented in the comment in layout.h for "NTFS_RECORD"
and also on MSDN for "MULTI_SECTOR_HEADER", the update sequence array
must end before the last le16 in the first logical sector --- not merely
before the end of the record.
Fix the validation and move it into a helper function, as it was done
identically in the read and write paths.
Signed-off-by: Eric Biggers <ebiggers3@gmail.com>
f86c4403ed