linux-kernel - Re: [PATCH v2] vfs: add NOFOLLOW flag to umount(2)

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Message-ID: <20100221020144.GV30031@ZenIV.linux.org.uk>
Date:	Sun, 21 Feb 2010 02:01:44 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Eugene Teo <eugene@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Bodo Eggert <7eggert@....de>
Subject: Re: [PATCH v2] vfs: add NOFOLLOW flag to umount(2)

On Thu, Feb 11, 2010 at 12:21:00PM -0500, Christoph Hellwig wrote:
> On Wed, Feb 10, 2010 at 12:15:53PM +0100, Miklos Szeredi wrote:
> >  - renamed flag to UMOUNT_NOFOLLOW
> >  - added UMOUNT_UNUSED for feature detection
> 
> Umm, why?  MNT_ certainly isn't the best naming for unmount flags,
> but switching convention after we had a few doesn't make any sense.

Actually, I've got more interesting question: what's being attempted
there?  Is that just a "let's protect ourselves against somebody feeding
us an untrusted symlink"?  I'm not sure if it makes much sense; if we
are dealing with pathnames on untrusted fs, there's nothing to stop the
attacker from having /mnt/foo/dir (originally containing a mountpoint
at /mnt/foo/dir/usr) killed and replaced with a symlink to /, making any
code that does umount() on such pathnames vulnerable as hell anyway.

Lack of LOOKUP_FOLLOW affects only the last pathname component.  So what
is that patch trying to make safe?
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