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

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Date:	Wed, 04 May 2011 13:55:33 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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 19:03 +0200, Frederic Weisbecker wrote:
> On Wed, May 04, 2011 at 12:22:40PM -0400, Eric Paris wrote:
> So the following set of operations:
> > 
> > SECCOMP_FILTER_SET, __NR_foo, "a=0"
> > SECCOMP_FILTER_SET, __NR_read, "1"
> > SECCOPM_FILTER_APPLY
> >  
> > SECCOMP_FILTER_SET, __NR_foo, "b=0"
> > SECCOMP_FILTER_APPLY
> > 
> > SECCOMP_FILTER_SET, __NR_write, "1"
> > SECCOMP_FILTER_APPLY
> > 
> > Would return EPERM for the __NR_write entry since it was a new syscall
> > after a set.  I think we agree on all this.
> 
> No, why?
> 
> The default filter for a syscall, if none have been given for it, is "0".
> 
> Thus, if you write "1" later, the entire filter is going to be:
> 
> 	"0 && 1"
> 
> Which is fine, we are not overriding already applied permissions there.
> 
> So where is the need to return -EPERM in such a specific case? Is it
> worth the corner case to check in the kernel, and to handle in userspace?
> And for what reason?

I assumed without looking at the code (always a bad idea) that he wasn't
going to explicitly create a rule with "0" and was going to implicitly
deny anything without a rule.  If there is an explicit "0" rule then you
are right, i don't see a need to deny the set operation in the kernel.
But if it is implicit in the non-existence of a filter then it should be
easy to tell userspace it isn't allowed any more.

-Eric

-Eric


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