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

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Date:	Sat, 28 Jan 2012 11:50:57 -0800
From:	Will Drewry <wad@...omium.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	linux-kernel@...r.kernel.org, keescook@...omium.org,
	john.johansen@...onical.com, serge.hallyn@...onical.com,
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Subject: Re: [PATCH v5 2/3] seccomp_filters: system call filtering using BPF

On Sat, Jan 28, 2012 at 8:28 AM, Andrew Lutomirski <luto@....edu> wrote:
> 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.

Good call.  I'll upgrade the interaction with no_new_privs to be
explicit in the next revision.  It is certainly more transparent and
removes the risk of unintended consequences.

thanks!
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