linux-kernel - Re: [RFC 0/4] make call_usermodehelper a bit more "safe"

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Message-ID: <20161220104846.GA18702@kroah.com>
Date:   Tue, 20 Dec 2016 11:48:46 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     NeilBrown <neilb@...e.com>, kernel-hardening@...ts.openwall.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/4] make call_usermodehelper a bit more "safe"

On Tue, Dec 20, 2016 at 11:31:57AM +0100, Jiri Kosina wrote:
> On Tue, 20 Dec 2016, Jiri Kosina wrote:
> 
> > I stay totally unconvinced that such kind of countermeasure brings any 
> > value whatsoever. Could you please bring up a particular usecase, where 
> > you have complete control over kernel memory, and still the only 
> > possible exploit factor is redirecting usermodhelper? It feels like 
> > rather random shot into darkness.
> 
> If we want to make usermod helper really secure, perhaps the best way to 
> go would be to completely nuke it and handle everyhting in udev; that'd be 
> quite some work though, especially so that we don't break all the corner 
> cases of module autoloading (request_module() and such).

In talking about this with others, I like Neil's approach of just
calling out to a statically-defined single binary to handle all of the
specifics.  Using something like busybox/toybox to handle any usermode
helper issues would be a very simple way to deal with this on a large
number of systems (i.e. embedded devices / phones / chromebooks).

After I return from vacation, I'll respin this series based on that idea
and repost it.

thanks,

greg k-h