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

[Heming Zhao]

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.
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.

- Heming

> > Add an explicit check for a NULL 'rec' and use an alternate error
> > message in that case to avoid unsafe access.
> >
> > Reported-by: syzbot+20282c1b2184a857ac4c@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://lore.kernel.org/all/67483b75.050a0220.253251.007c.GAE@xxxxxxxxxx/T/
> > Signed-off-by: Ivan Pravdin <ipravdin.official@xxxxxxxxx>
> > ---
> >   fs/ocfs2/dir.c | 16 +++++++++++-----
> >   1 file changed, 11 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/ocfs2/dir.c b/fs/ocfs2/dir.c
> > index 7799f4d16ce9..dccf0349e523 100644
> > --- a/fs/ocfs2/dir.c
> > +++ b/fs/ocfs2/dir.c
> > @@ -809,11 +809,17 @@ static int ocfs2_dx_dir_lookup_rec(struct inode *inode,
> >   	}
> >   
> >   	if (!found) {
> > -		ret = ocfs2_error(inode->i_sb,
> > -				  "Inode %lu has bad extent record (%u, %u, 0) in btree\n",
> > -				  inode->i_ino,
> > -				  le32_to_cpu(rec->e_cpos),
> > -				  ocfs2_rec_clusters(el, rec));
> > +		if (rec) {
> > +			ret = ocfs2_error(inode->i_sb,
> > +					"Inode %lu has bad extent record (%u, %u, 0) in btree\n",
> > +					inode->i_ino,
> > +					le32_to_cpu(rec->e_cpos),
> > +					ocfs2_rec_clusters(el, rec));
> > +		} else {
> > +			ret = ocfs2_error(inode->i_sb,
> > +					"Inode %lu has no extent records in btree\n",
> > +					inode->i_ino);
> > +		}
> >   		goto out;
> >   	}
> >