linux-kernel - Re: [PATCH v5 2/3] seccomp_filters: system call filtering using BPF

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Message-ID: <CAObL_7FAh27PU5koksh--v5yE9N7abz26=MQJ8np1U0PQDnP-w@mail.gmail.com>
Date:	Sat, 28 Jan 2012 08:28:09 -0800
From:	Andrew Lutomirski <luto@....edu>
To:	Will Drewry <wad@...omium.org>
Cc:	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,
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	mhalcrow@...gle.com, dlaor@...hat.com, corbet@....net,
	alan@...rguk.ukuu.org.uk, indan@....nu, mcgrathr@...omium.org
Subject: Re: [PATCH v5 2/3] seccomp_filters: system call filtering using BPF

On Fri, Jan 27, 2012 at 3:24 PM, Will Drewry <wad@...omium.org> wrote:
> Filter programs will be inherited across fork/clone and execve.
> However, if the task attaching the filter is unprivileged
> (!CAP_SYS_ADMIN) the no_new_privs bit will be set on the task.  This
> ensures that unprivileged tasks cannot attach filters that affect
> privileged tasks (e.g., setuid binary).

This makes me nervous -- I don't think that the behavior of any new
API should be different depending on privilege level -- adding a
privilege should just make things work that would otherwise fail.  You
might end up with bugs where a program is completely safe if run
without CAP_SYS_ADMIN but, if run with CAP_SYS_ADMIN, bad things
happen.  (The behavior of setuid(geteuid()) is an example of this
problem.)

One way to fix it is to make setting a filter program fail unless
capable(CAP_SYS_ADMIN ) || no_new_privs.

--Andy
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