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

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Date:   Tue, 20 Dec 2016 11:27:13 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.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, 20 Dec 2016, Greg KH wrote:

> > Sorry, I really don't get this.
> > 
> > If kernel memory can be easily changed (which is assumed here), why bother 
> > with all this? I'll just set current->uid to 0 and be done.
> 
> Because you don't want your current process to uid 0, you want some
> other program to run as root.  It's quite common for exploits to work
> this way, take a look at how the p0wn-to-own "contests" usually break
> out of sandboxed systems like browsers.

So what kind of sandbox are we talking about here?

namespaces-based sanbox? If you have direct access to kernel memory, you 
can just assign whatever context you want to task_struct's fs struct, and 
you are out of a sandbox.

chroot-based sandbox? Exactly the same argument (you just reset fs->root 
in task_struct).

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.

Thanks,

-- 
Jiri Kosina
SUSE Labs