linux-kernel - Re: [PATCH] hfs: Fix OOB Write in hfs

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Date:   Tue, 29 Nov 2022 10:23:46 +0800
From:   "zhangpeng (AS)" <zhangpeng362@...wei.com>
To:     Viacheslav Dubeyko <slava@...eyko.com>
CC:     <zippel@...ux-m68k.org>, <akpm@...l.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <sunnanyong@...wei.com>,
        <wangkefeng.wang@...wei.com>,
        <syzbot+dc3b1cf9111ab5fe98e7@...kaller.appspotmail.com>
Subject: Re: [PATCH] hfs: Fix OOB Write in hfs_asc2mac

On 2022/11/29 3:29, Viacheslav Dubeyko wrote:

>> On Nov 25, 2022, at 8:36 PM, Peng Zhang <zhangpeng362@...wei.com> wrote:
>>
>> From: ZhangPeng <zhangpeng362@...wei.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@...kaller.appspotmail.com
>> Signed-off-by: ZhangPeng <zhangpeng362@...wei.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.

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>
}


Thanks,
Zhang Peng

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