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

[Viacheslav Dubeyko]

> On Nov 28, 2022, at 6:23 PM, zhangpeng (AS) <zhangpeng362@xxxxxxxxxx> wrote:
> 
> On 2022/11/29 3:29, Viacheslav Dubeyko wrote:
> 
>>> On Nov 25, 2022, at 8:36 PM, Peng Zhang <zhangpeng362@xxxxxxxxxx> wrote:
>>> 
>>> From: ZhangPeng <zhangpeng362@xxxxxxxxxx>
>>> 
>>> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>>> Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx>
>>> ---
>>> 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.
> 
> Thank you for your help.
> 
> After testing, it fix the issue.
> Would it be better to add dstlen > 0 instead of replacing srclen > 0 with dstlen > 0?
> Because there may be dstlen > 0 and srclen <= 0.
> 
> we can use:
> 
> while (srclen > 0 && dstlen > 0) {
>   <skipped>
> } else {
>   <skipped>
> }
> 

Looks good to me.

Thanks,
Slava.

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