linux-kernel - Re: [PATCH 3/5] v2 seccomp_filters: Enable ftrace-based system call filtering

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Message-ID: <20110526194621.GB6363@elte.hu>
Date:	Thu, 26 May 2011 21:46:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	david@...g.hm
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Drewry <wad@...omium.org>,
	Colin Walters <walters@...bum.org>,
	Kees Cook <kees.cook@...onical.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, James Morris <jmorris@...ei.org>
Subject: Re: [PATCH 3/5] v2 seccomp_filters: Enable ftrace-based system call
 filtering


* david@...g.hm <david@...g.hm> wrote:

> On Thu, 26 May 2011, Ingo Molnar wrote:
> 
> >* Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> >
> >>It also gets rid of all configuration - one of the things that
> >>makes most security frameworks (look at selinux, but also just
> >>ACL's etc) such a crazy rats nest is the whole "set up for other
> >>processes". If it's designed very much to be about just the "self"
> >>process (after initialization etc), then I think that avoids pretty
> >>much all the serious issues.
> >
> >That's how the event filters work currently: even when inherited they
> >get removed when exec-ing a setuid task, so they cannot leak into
> >privileged context and cannot modify execution there.
> >
> >Inheritance works when requested, covering only same-credential child
> >tasks, not privileged successors.
> 
> this is a very reasonable default, but there should be some way of 
> saying that you want the restrictions to carry over to the suid 
> task (I really know what I'm doing switch)

Unless you mean that root should be able to do it it's a security 
hole both for events and for filters:

 - for example we dont want really finegrained events to be used to 
   BTS hw-trace sshd and thus enable it to discover cryptographic 
   properties of the private key sshd is using.

 - we do not want to *modify* the execution flow of a setuid program,
   that can lead to exploits: by pushing the privileged codepath into 
   a condition that can never occur on a normal system - and thus can 
   push it into doing something it was not intended to do.

   data damage could be done as well: for example if the privileged 
   code is logging into a system file then modifying execution can 
   damage the log file.

So it's not a good idea in general to allow unprivileged code to 
modify the execution of privileged code. In fact it's not a good idea 
to allow it to simply *observe* privileged code. It must remain a 
black box with very few information leaking outwards.

Thanks,

	Ingo
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