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

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Message-ID: <aRJvXWcwkUeal7DO@Bertha>
Date: Mon, 10 Nov 2025 23:03:57 +0000
From: George Anthony Vernon <contact@...rnon.com>
To: Viacheslav Dubeyko <Slava.Dubeyko@....com>
Cc: "glaubitz@...sik.fu-berlin.de" <glaubitz@...sik.fu-berlin.de>,
	"slava@...eyko.com" <slava@...eyko.com>,
	"frank.li@...o.com" <frank.li@...o.com>,
	"skhan@...uxfoundation.org" <skhan@...uxfoundation.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel-mentees@...ts.linux.dev" <linux-kernel-mentees@...ts.linux.dev>,
	"penguin-kernel@...ove.sakura.ne.jp" <penguin-kernel@...ove.sakura.ne.jp>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"syzbot+97e301b4b82ae803d21b@...kaller.appspotmail.com" <syzbot+97e301b4b82ae803d21b@...kaller.appspotmail.com>
Subject: Re: [PATCH v2 2/2] hfs: Update sanity check of the root record

On Tue, Nov 04, 2025 at 11:01:31PM +0000, Viacheslav Dubeyko wrote:
> 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@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=97e301b4b82ae803d21b  
> > Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> > Signed-off-by: George Anthony Vernon <contact@...rnon.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]:
> 
> The hfs_iget() calls iget5_locked() [2]:
> 
> 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.
> 

I think Tetsuo's concern is that a directory catalog record with
cnid > 15 might be returned as a result of hfs_bnode_read, which
is_valid_cnid() would not protect against. I've satisfied myself that
hfs_bnode_read() in hfs_fill_super() will populate hfs_find_data fd
correctly and crash out if it failed to find a record with root CNID so
this path is unreachable and there is no need for the second 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;

Agreed, I see hfs_read_inode might return a bad inode. Thanks for
catching this. I noticed also that it returns an int but the return
value holds no meaning; it is always zero.

> Thanks,
> Slava.
>

Many thanks again,

George