hfs: Fix OOB Write in hfs_asc2mac

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/[email protected]
Fixes: 328b922 ("[PATCH] hfs: NLS support")
Signed-off-by: ZhangPeng <[email protected]>
Reviewed-by: Viacheslav Dubeyko <[email protected]>
Reported-by: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ZhangPeng

fs/hfs/trans.c

@@ -109,7 +109,7 @@ void hfs_asc2mac(struct super_block *sb, struct hfs_name *out, const struct qstr
 	if (nls_io) {
 		wchar_t ch;
 
-		while (srclen > 0) {
+		while (srclen > 0 && dstlen > 0) {
 			size = nls_io->char2uni(src, srclen, &ch);
 			if (size < 0) {
 				ch = '?';