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

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Message-ID: <CA+55aFw2yigXqfiKMH4aZ9w8dOkAEB0cZ2m4ZUEhiddnrQuueg@mail.gmail.com>
Date:	Thu, 12 Jan 2012 09:09:31 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andrew Lutomirski <luto@....edu>, 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 8:27 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> In that case, just have execv fail if filtering is enabled and we are
> execing a setuid program. But I don't see why non "magical" execv's
> should be prohibited.

The whole "fail security escalations" thing goes way beyond just
filtering, I think we could seriously try to make it a generic
feature.

For example, somebody just asked me the other day why "chroot()"
requires admin privileges, since it would be good to limit even
non-root things.

And it's really the exact same issue as filtering: in some sense,
chroot() "filters" FS name lookups, and can be used to fool programs
that are written to be secure.

We could easily introduce a per-process flag that just says "cannot
escalate privileges". Which basically just disables execve() of
suid/sgid programs (and possibly other things too), and locks the
process to the current privileges. And then make the rule be that *if*
that flag is set, you can then filter across an execve, or chroot as a
normal user, or whatever.

There are probably other things like that - things like allowing users
to do bind mounts etc - that aren't dangerous in themselves, but that
are dangerous mainly because they can be used to fool things into
privilege escalations. So this is definitely not a filter-only issue.

                       Linus
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