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

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Message-ID: <CABqD9hbOUy1qO0f+JFitRXH6c5EgLTWOh5eGdo8dTxeXJ40h2g@mail.gmail.com>
Date:	Thu, 12 Jan 2012 10:59:15 -0600
From:	Will Drewry <wad@...omium.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, keescook@...omium.org,
	john.johansen@...onical.com, serge.hallyn@...onical.com,
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Subject: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

On Thu, Jan 12, 2012 at 9:43 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, 2012-01-11 at 11:25 -0600, Will Drewry wrote:
>
>> Filter programs may _only_ cross the execve(2) barrier if last filter
>> program was attached by a task with CAP_SYS_ADMIN capabilities in its
>> user namespace.  Once a task-local filter program is attached from a
>> process without privileges, execve will fail.  This ensures that only
>> privileged parent task can affect its privileged children (e.g., setuid
>> binary).
>
> This means that a non privileged user can not run another program with
> limited features? How would a process exec another program and filter
> it? I would assume that the filter would need to be attached first and
> then the execv() would be performed. But after the filter is attached,
> the execv is prevented?

Yeah - it means tasks can filter themselves, but not each other.
However, you can inject a filter for any dynamically linked executable
using LD_PRELOAD.

> Maybe I don't understand this correctly.

You're right on.  This was to ensure that one process didn't cause
crazy behavior in another. I think Alan has a better proposal than
mine below.  (Goes back to catching up.)
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