Re: [PATCH v4] hfs: update sanity check of the root record

[Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>]

On Wed, 2025-07-30 at 08:21 +0900, Tetsuo Handa 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>
> ---
>  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 fe09c2093a93..d231989b4e23 100644
> --- a/fs/hfs/super.c
> +++ b/fs/hfs/super.c
> @@ -354,7 +354,7 @@ 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))
>  			res = -EIO;
>  	}
>  	if (res)

Why do not localize the all checks in hfs_read_inode()?

We will do such logic then [1], even if rec.dir.DirID !=
cpu_to_be32(HFS_ROOT_CNID):

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]:

inode = iget5_locked(sb, cnid, hfs_test_inode, hfs_read_inode, &data);

And hfs_read_inode() will be called, finally. If inode ID is wrong, then
make_bad_inode(inode) can be called [3].

If we considering case HFS_CDR_DIR in hfs_read_inode(), then we know that it
could be HFS_POR_CNID, HFS_ROOT_CNID, or >= HFS_FIRSTUSER_CNID. Do you mean that
HFS_POR_CNID could be a problem in hfs_write_inode()?

Thanks,
Slava.

[1] https://elixir.bootlin.com/linux/v6.16-rc6/source/fs/hfs/super.c#L363
[2] https://elixir.bootlin.com/linux/v6.16-rc6/source/fs/hfs/inode.c#L403
[3] 
https://elixir.bootlin.com/linux/v6.16-rc6/source/fs/hfs/inode.c#L373