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

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Date:	Thu, 12 Jan 2012 12:40:06 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jamie Lokier <jamie@...reable.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, 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,
	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, 2012-01-12 at 17:30 +0000, Jamie Lokier wrote:

> You can do this now, using ptrace().  It's horrible, but half of the
> horribleness is needing to understand machine-dependent registers,
> which this new patch doesn't address.  (The other half is a ton of
> undocumented but important ptrace() behaviours on Linux.)

Yeah I know the horrid use of ptrace, I've implemented programs that use
it :-p

I guess ptrace can capture the execv and determine if it is OK or not to
run it. But again, this doesn't stop the possible attacks that could
happen, with having the execv on a symlink file, having the ptrace check
say its OK, and then switching the symlink to a setuid file.

When the new execv executed, the parent process would lose all control
over it. The idea is to prevent this.

I like Alan's suggestion. Have userspace decide to allow execv or not,
and even let it decide if it should allow setuid execv's or not, but
still allow non-setuid execvs. If you allow the setuid execv, once that
happens, the same behavior will occur as with ptrace. A setuid execv
will lose all its filtering.

-- Steve


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