Re: [PATCH] hfs: Fix OOB Write in hfs_asc2mac

[Viacheslav Dubeyko <slava@dubeyko.com>]

> On Nov 25, 2022, at 8:36 PM, Peng Zhang <zhangpeng362@huawei.com> wrote:
> 
> From: ZhangPeng <zhangpeng362@huawei.com>
> 
> 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 before Writing to dst address.
> 
> Fixes: 328b92278650 ("[PATCH] hfs: NLS support")
> Reported-by: syzbot+dc3b1cf9111ab5fe98e7@syzkaller.appspotmail.com
> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
> ---
> fs/hfs/trans.c | 2 ++
> 1 file changed, 2 insertions(+)
> 
> diff --git a/fs/hfs/trans.c b/fs/hfs/trans.c
> index 39f5e343bf4d..886158db07b3 100644
> --- a/fs/hfs/trans.c
> +++ b/fs/hfs/trans.c
> @@ -130,6 +130,8 @@ void hfs_asc2mac(struct super_block *sb, struct hfs_name *out, const struct qstr
> 				dst += size;
> 				dstlen -= size;
> 			} else {
> +				if (dstlen == 0)
> +					goto out;

Maybe, it makes sense to use dstlen instead of srclen in while()?

We have now:

while (srclen > 0) {
   <skipped>
} else {
   <skipped>
}

We can use instead:

while (dstlen > 0) {
   <skipped>
} else {
   <skipped>
}

Will it fix the issue?

Thanks,
Slava.


> 				*dst++ = ch > 0xff ? '?' : ch;
> 				dstlen--;
> 			}
> -- 
> 2.25.1
>