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

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Message-ID: <20120112174433.GM7180@jl-vm1.vm.bytemark.co.uk>
Date:	Thu, 12 Jan 2012 17:44:33 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Steven Rostedt <rostedt@...dmis.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

Steven Rostedt wrote:
> 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

That warm fuzzy feeling :-)

> 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.

fexecve() exists to solve the problem.
Also known as execve("/proc/self/fd/...") on Linux.

> 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.

I like the idea of letting the tracer decide what it wants.

-- Jamie
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