Syzbot reported a OOB Write bug:

loop0: detected capacity change from 0 to 64
==================================================================
BUG: KASAN: slab-out-of-bounds in hfs_asc2mac+0x467/0x9a0
fs/hfs/trans.c:133
Write of size 1 at addr ffff88801848314e by task syz-executor391/3632

Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0x1b1/0x28e lib/dump_stack.c:106
 print_address_description+0x74/0x340 mm/kasan/report.c:284
 print_report+0x107/0x1f0 mm/kasan/report.c:395
 kasan_report+0xcd/0x100 mm/kasan/report.c:495
 hfs_asc2mac+0x467/0x9a0 fs/hfs/trans.c:133
 hfs_cat_build_key+0x92/0x170 fs/hfs/catalog.c:28
 hfs_lookup+0x1ab/0x2c0 fs/hfs/dir.c:31
 lookup_open fs/namei.c:3391 [inline]
 open_last_lookups fs/namei.c:3481 [inline]
 path_openat+0x10e6/0x2df0 fs/namei.c:3710
 do_filp_open+0x264/0x4f0 fs/namei.c:3740

If in->len is much larger than HFS_NAMELEN(31) which is the maximum
length of an HFS filename, a OOB write could occur in hfs_asc2mac(). In
that case, when the dst reaches the boundary, the srclen is still
greater than 0, which causes a OOB write.
Fix this by adding a check on dstlen in while() before writing to dst
address.

Link: https://lkml.kernel.org/r/20221202030038.1391945-1-zhangpeng362@huawei.com
Fixes: 328b9227 ("[PATCH] hfs: NLS support")
Signed-off-by: NZhangPeng <zhangpeng362@huawei.com>
Reviewed-by: NViacheslav Dubeyko <slava@dubeyko.com>
Reported-by: <syzbot+dc3b1cf9111ab5fe98e7@syzkaller.appspotmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>