linux-kernel - Re: [PATCH] fs: Prevent non-symlinks from entering pick link

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Message-ID: <20250618050200.GP1880847@ZenIV>
Date: Wed, 18 Jun 2025 06:02:00 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Edward Adam Davis <eadavis@...com>
Cc: syzbot+1aa90f0eb1fc3e77d969@...kaller.appspotmail.com,
	almaz.alexandrovich@...agon-software.com, brauner@...nel.org,
	jack@...e.cz, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, ntfs3@...ts.linux.dev,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] fs: Prevent non-symlinks from entering pick link

On Wed, Jun 18, 2025 at 05:50:16AM +0100, Al Viro wrote:

> NAK.  This is not the first time that garbage is suggested and no,
> we are not going to paper over that shite in fs/namei.c.
> 
> Not going to happen.
> 
> You ARE NOT ALLOWED to call make_bad_inode() on a live inode, period.
> Never, ever to be done.
> 
> There's a lot of assertions it violates and there's no chance in
> hell to plaster each with that kind of checks.
> 
> Fix NTFS.  End of story.

To elaborate a bit: if you look at the end of e.g. their attr_set_size(),
you'll see
out:
        if (is_bad) {
bad_inode:
		_ntfs_bad_inode(&ni->vfs_inode);
	}
	return err;
}

This is a bug.  So are similar places all over the place there.
You are not supposed to use make_bad_inode() as a general-purpose
"something went wrong, don't wanna see it anymore" tool.

And as long as it stays there, any fuzzing reports of ntfs are pretty
much worthless - any of those places (easily located by grepping for
_ntfs_bad_inode) can fuck the kernel up.  Once ntfs folks get around
to saner error recovery, it would make sense to start looking into
fuzzing that thing again.  Until then - nope.  Again, this is *NOT*
going to be papered over in a random set of places (pretty certain
to remain incomplete) in VFS.