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

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Date:	Thu, 26 May 2011 12:49:15 -0700 (PDT)
From:	david@...g.hm
To:	Ingo Molnar <mingo@...e.hu>
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

On Thu, 26 May 2011, Ingo Molnar wrote:

> * 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.

I was thinking of the use case of the real sysadmin (i.e. root) wanting to 
be able to constrain things. I can see why you would not want to allow 
normal users to do this.

David Lang
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