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

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Date:	Thu, 12 Jan 2012 17:36:17 +0000
From:	Jamie Lokier <jamie@...reable.org>
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, luto@....edu,
	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

Steven Rostedt 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?

Ugly method: Using ptrace(), trap after the execve() and issue fake
syscalls to install the filter.  I feel dirty thinking it, in a good way.

LD_PRELOAD has been suggested.  It's not 100% reliable because not all
executables are dynamic (on some uClinux platforms none of them are),
but it will usually work.

-- Jamie
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