linux-kernel - Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

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Message-ID: <CAObL_7GVo8rH=Knv=t5VtJG24m7tY9MhNf9QNah+8G9oYiXo6g@mail.gmail.com>
Date:	Thu, 12 Jan 2012 08:14:25 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	torvalds@...ux-foundation.org, segoon@...nwall.com,
	jmorris@...ei.org, scarybeasts@...il.com, avi@...hat.com,
	penberg@...helsinki.fi, viro@...iv.linux.org.uk, mingo@...e.hu,
	akpm@...ux-foundation.org, khilman@...com, borislav.petkov@....com,
	amwang@...hat.com, oleg@...hat.com, ak@...ux.intel.com,
	eric.dumazet@...il.com, gregkh@...e.de, dhowells@...hat.com,
	daniel.lezcano@...e.fr, linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com
Subject: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

On Thu, Jan 12, 2012 at 7:43 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, 2012-01-11 at 11:25 -0600, Will Drewry wrote:
>
>> Filter programs may _only_ cross the execve(2) barrier if last filter
>> program was attached by a task with CAP_SYS_ADMIN capabilities in its
>> user namespace.  Once a task-local filter program is attached from a
>> process without privileges, execve will fail.  This ensures that only
>> privileged parent task can affect its privileged children (e.g., setuid
>> binary).
>
> This means that a non privileged user can not run another program with
> limited features? How would a process exec another program and filter
> it? I would assume that the filter would need to be attached first and
> then the execv() would be performed. But after the filter is attached,
> the execv is prevented?
>
> Maybe I don't understand this correctly.

Time to resurrect execve_nosecurity?  If so, then the rule could be
simplified to: seccomp programs cannot use normal execve at all.

The longer I linger on lists and see neat ideas like this, the more I
get annoyed that execve is magical.  I dream of a distribution that
doesn't use setuid, file capabilities, selinux transitions on exec, or
any other privilege changes on exec *at all*.  I think that the only
things missing in the kernel (other than something intelligent to do
about SELinux) are execve_nosecurity and the ability for a normal
program to wait for an unrelated program to finish (or some other way
that a program can ask a daemon to spawn a privileged program for it
and then to cleanly wait for that program to finish in a way that
could survive re-exec of the daemon).

--Andy

>
> -- Steve
>
>
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