linux-kernel - Re: [PATCH 5/7] seccomp_filter: Document what seccomp

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Date:	Wed, 04 May 2011 12:06:09 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Eric Paris <eparis@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Will Drewry <wad@...omium.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, kees.cook@...onical.com,
	agl@...omium.org, jmorris@...ei.org,
	Randy Dunlap <rdunlap@...otime.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tom Zanussi <tzanussi@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 5/7] seccomp_filter: Document what seccomp_filter is
 and how it works.

On Wed, 2011-05-04 at 11:54 -0400, Eric Paris wrote:

> As this is a deny by default interface which only allows you to further
> restrict you couldn't add more than 1 syscall if you didn't have an
> explict 'apply' action.
> 
> SECCOMP_FILTER_SET, __NR_fo, "a=0"
> SECCOMP_FILTER_SET, __NR_read, "1" == EPERM
> 
> Maybe apply on set is fine after the first apply, but we definitely need
> some way to do more than 1 set before the rules are applied....

So we could have SET be 'or' and APPLY be 'and'.

SECCOMP_FILTER_SET, __NR_foo, "a=0"
SECCOMP_FILTER_SET, __NR_read, "1" == EPERM
SECCOPM_FILTER_APPLY

SECCOMP_FILTER_SET, __NR_foo, "b=0"
SECCOPM_FILTER_APPLY

Will end up being:

(foo: a == 0 || read: "1" == EPERM) && (foo: b == 0)

The second set/apply now removes the read option, and foo only works if
a is 0 and b is 0.

This would also work for children, as they can only restrict (with
'and') and can not add more control.

-- Steve


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