linux-kernel - Re: [PATCH] ocfs2: Avoid NULL pointer dereference in dx_dir_lookup

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Message-ID: <d049aef6-4dc1-4a20-a8e3-306fd3e3b63a@suse.com>
Date: Mon, 30 Jun 2025 15:23:07 +0800
From: Heming Zhao <heming.zhao@...e.com>
To: Joseph Qi <joseph.qi@...ux.alibaba.com>,
 Ivan Pravdin <ipravdin.official@...il.com>, mark@...heh.com,
 jlbec@...lplan.org, ocfs2-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc: syzbot+20282c1b2184a857ac4c@...kaller.appspotmail.com
Subject: Re: [PATCH] ocfs2: Avoid NULL pointer dereference in
 dx_dir_lookup_rec()

On 6/30/25 14:58, Joseph Qi wrote:
> 
> 
> On 2025/6/30 10:32, Heming Zhao wrote:
>> On 6/30/25 09:26, Joseph Qi wrote:
>>> Hi,
>>>
>>>
>>> On 2025/6/27 10:38, Ivan Pravdin wrote:
>>>> When a directory entry is not found, ocfs2_dx_dir_lookup_rec() prints an
>>>> error message that unconditionally dereferences the 'rec' pointer.
>>>> However, if 'rec' is NULL, this leads to a NULL pointer dereference and
>>>> a kernel panic.
>>>>
>>>
>>> This looks possible, but syzbot reports slab-out-of-bounds Read in
>>> ocfs2_dx_dir_lookup_rec(), not NULL pointer dereference.
>>>
>>> So I think it is because it construct a malicious image and set a wrong
>>> l_recs, then access this damaged l_recs.
>>>
>>> Thanks,
>>> Joseph
>>
>> I think this proposed fix (at least the fix method) is acceptable.
> 
> But this seems different from the issue syzbot reports, right?
> https://lore.kernel.org/all/67483b75.050a0220.253251.007c.GAE@google.com/T/
> 
> Thanks,
> Joseph

OK, I think I may have found the problem. In the URL above,
there are two issues/reports:
1> From: syzbot @ 2024-11-28  9:44 UTC
2> From: syzbot @ 2025-03-09 11:40 UTC

Your review comments about report <1>; Ivan's patch is about report <2>.

And the dashboard shows the <2>:
- https://syzkaller.appspot.com/bug?extid=20282c1b2184a857ac4c

It seems syzbot changed the fuzz code and won't trigger issue <1>.

For issue <1>, the rec was an illegal value, and made KASAN report a bug.

> 
>> the crash occurs at ocfs2_error(), where the pointer 'rec' must be incorrect.
>> look back at the previous code lines:
>>          for (i = le16_to_cpu(el->l_next_free_rec) - 1; i >= 0; i--) {
>>                  rec = &el->l_recs[i];
>>
>>                  if (le32_to_cpu(rec->e_cpos) <= major_hash) {
>>                          found = 1;
>>                          break;
>>                  }
>>          }
>>
>> either 'el->l_next_free_rec' or 'el->l_recs[i]' has an incorrect value.
>> we can do nothing about this kind of error, simply returning errno to caller is sufficient.
>>
>> btw, ocfs2-tools has a similar function "static errcode_t ocfs2_dx_dir_lookup_rec()"@libocfs2/dir_indexed.c, which sets ret with OCFS2_ET_CORRUPT_EXTENT_BLOCK and return.
>>
> 
> 
>