Re: [PATCH] ocfs2: Avoid NULL pointer dereference in dx_dir_lookup_rec()

[Heming Zhao]

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@xxxxxxxxxx/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.