linux-kernel - Re: [PATCH v4] Add a "nosymfollow" mount option.

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Date:   Sun, 9 Feb 2020 20:12:36 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Ross Zwisler <zwisler@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Ross Zwisler <zwisler@...omium.org>,
        linux-kernel@...r.kernel.org,
        Mattias Nissler <mnissler@...omium.org>,
        Benjamin Gordon <bmgordon@...gle.com>,
        Raul Rangel <rrangel@...gle.com>,
        Micah Morton <mortonm@...gle.com>,
        Dmitry Torokhov <dtor@...gle.com>, Jan Kara <jack@...e.cz>,
        David Howells <dhowells@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4] Add a "nosymfollow" mount option.

On 2020-02-03, Ross Zwisler <zwisler@...gle.com> wrote:
> On Sat, Feb 01, 2020 at 05:27:44PM +1100, Aleksa Sarai wrote:
> > On 2020-01-31, Ross Zwisler <zwisler@...gle.com> wrote:
> > > On Fri, Jan 31, 2020 at 12:51:34PM +1100, Aleksa Sarai wrote:
> > > If noxdev would involve a pathname traversal to make sure you don't ever leave
> > > mounts with noxdev set, I think this could potentially cover the use cases I'm
> > > worried about.  This would restrict symlink traversal to files within the same
> > > filesystem, and would restrict traversal to both normal and bind mounts from
> > > within the restricted filesystem, correct?
> > 
> > Yes, but it would have to block all mountpoint crossings including
> > bind-mounts, because the obvious way of checking for mountpoint
> > crossings (vfsmount comparisons) results in bind-mounts being seen as
> > different mounts. This is how LOOKUP_NO_XDEV works. Would this be a
> > show-stopped for ChromeOS?
> >
> > I personally find "noxdev" to be a semantically clearer statement of
> > intention ("I don't want any lookup that reaches this mount-point to
> > leave") than "nosymfollow" (though to be fair, this is closer in
> > semantics to the other "no*" mount flags). But after looking at [1] and
> > thinking about it for a bit, I don't really have a problem with either
> > solution.
> 
> For ChromeOS we want to protect data both on user-provided filesystems (i.e.
> USB attached drives and the like) as well as on our "stateful" partition.  
> 
> The noxdev mount option would resolve our concerns for user-provided
> filesystems, but I don't think that we would be able to use it for stateful
> because symlinks on stateful that point elsewhere within stable are still a
> security risk.  There is more explanation on why this is the case in [1].
> Thank you for linking to that, by the way.
> 
> I think our security concerns around both use cases, user-provided filesystems
> and the stateful partition, can be resolved in ChromeOS with the nosymfollow
> mount flag.  Based on that, my current preference is for the 'nosymfollow'
> mount flag.

Fair enough. I can work on and send "noxdev" separately -- I only
brought it up because the attack scenarios (and connection to openat2)
are both fairly similar.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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