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

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Date:	Thu, 12 Jan 2012 10:32:59 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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,
	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 10:18 AM, Andrew Lutomirski <luto@....edu> wrote:
>
> Like this?
>
> http://lkml.indiana.edu/hypermail/linux/kernel/1003.3/01225.html

I don't know the execve_nosecurity patches, so the diff makes little
sense to me, but yeah, I wouldn't expect it to be more than a couple
of lines. Exactly *how* you set the bit etc is not something I care
deeply about, prctl seems about as good as anything.

> Note that there's a huge can of worms if execve is allowed but
> suid/sgid is not: selinux may elevate privileges on exec of pretty
> much anything.  (I think that this is a really awful idea, but it's in
> the kernel, so we're stuck with it.)

You can do any amount of crazy things with selinux, but the other side
of the coin is that it would also be trivial to teach selinux about
this same "restricted environment" bit, and just say that a process
with that bit set doesn't get to match whatever selinux privilege
escalation rules..

I really don't think this is just about "execve cannot do setuid". I
think it's about the process being marked as restricted.

So in your patch, I think that "PR_RESTRICT_EXEC" bit is wrong. It
should simply be "PR_RESTRICT_ME", and be done with it, and not try to
artificially limit it to be some "execve feature", and more think of
it as a "this is a process that has *no* extra privileges at all, and
can never get them".

                            Linus
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