Re: [PATCH v2 2/2] hfs: Update sanity check of the root record

Viacheslav Dubeyko
2025-11-04T23:01:36Z

On Tue, 2025-11-04 at 01:47 +0000, George Anthony Vernon wrote:
> syzbot is reporting that BUG() in hfs_write_inode() fires upon unmount
> operation when the inode number of the record retrieved as a result of
> hfs_cat_find_brec(HFS_ROOT_CNID) is not HFS_ROOT_CNID, for commit
> b905bafdea21 ("hfs: Sanity check the root record") checked the record
> size and the record type but did not check the inode number.
>
> Reported-by: syzbot+97e301b4b82ae803d21b@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=97e301b4b82ae803d21b
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Signed-off-by: George Anthony Vernon <contact@gvernon.com>
> ---
> fs/hfs/super.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/hfs/super.c b/fs/hfs/super.c
> index 47f50fa555a4..a7dd20f2d743 100644
> --- a/fs/hfs/super.c
> +++ b/fs/hfs/super.c
> @@ -358,7 +358,7 @@ static int hfs_fill_super(struct super_block *sb, struct fs_context *fc)
> goto bail_hfs_find;
> }
> hfs_bnode_read(fd.bnode, &rec, fd.entryoffset, fd.entrylength);
> - if (rec.type != HFS_CDR_DIR)
> + if (rec.type != HFS_CDR_DIR || rec.dir.DirID != cpu_to_be32(HFS_ROOT_CNID))

This check is completely unnecessary. Because, we have hfs_iget() then [1]:

res = hfs_find_init(HFS_SB(sb)->cat_tree, &fd);
if (res)
goto bail_no_root;
res = hfs_cat_find_brec(sb, HFS_ROOT_CNID, &fd);
if (!res) {
if (fd.entrylength != sizeof(rec.dir)) {
res = -EIO;
goto bail_hfs_find;
}
hfs_bnode_read(fd.bnode, &rec, fd.entryoffset, fd.entrylength);
if (rec.type != HFS_CDR_DIR)
res = -EIO;
}
if (res)
goto bail_hfs_find;
res = -EINVAL;
root_inode = hfs_iget(sb, &fd.search_key->cat, &rec);
hfs_find_exit(&fd);
if (!root_inode)
goto bail_no_root;

The hfs_iget() calls iget5_locked() [2]:

struct inode *hfs_iget(struct super_block *sb, struct hfs_cat_key *key,
hfs_cat_rec *rec)
{
struct hfs_iget_data data = { key, rec };
struct inode *inode;
u32 cnid;

switch (rec->type) {
case HFS_CDR_DIR:
cnid = be32_to_cpu(rec->dir.DirID);
break;
case HFS_CDR_FIL:
cnid = be32_to_cpu(rec->file.FlNum);
break;
default:
return NULL;
}
inode = iget5_locked(sb, cnid, hfs_test_inode, hfs_read_inode, &data);
if (inode && (inode->i_state & I_NEW))
unlock_new_inode(inode);
return inode;
}

And iget5_locked() calls hfs_read_inode(). And hfs_read_inode() will call
is_valid_cnid() after applying your patch. So, is_valid_cnid() in
hfs_read_inode() can completely manage the issue. This is why we don't need in
this modification after your first patch.

But I think we need to check that root_inode is not bad inode afterwards:

root_inode = hfs_iget(sb, &fd.search_key->cat, &rec);
hfs_find_exit(&fd);
if (!root_inode || is_bad_inode(root_inode))
goto bail_no_root;

Thanks,
Slava.

> res = -EIO;
> }
> if (res)

[1] https://elixir.bootlin.com/linux/v6.18-rc4/source/fs/hfs/super.c#L367
[2] https://elixir.bootlin.com/linux/v6.18-rc4/source/fs/hfs/inode.c#L414