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

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Message-ID: <a259f224-a471-45e3-84de-7071ec6595e9@linux.alibaba.com>
Date: Mon, 30 Jun 2025 14:58:01 +0800
From: Joseph Qi <joseph.qi@...ux.alibaba.com>
To: Heming Zhao <heming.zhao@...e.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 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

> 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.
>