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Message-ID: <1304526163.10692.33.camel@localhost.localdomain>
Date: Wed, 04 May 2011 12:22:40 -0400
From: Eric Paris <eparis@...hat.com>
To: Steven Rostedt <rostedt@...dmis.org>
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 12:06 -0400, Steven Rostedt wrote:
> 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
When I said "== EPERM" I meant that the given prctl call would return
EPERM. I'm going to pretend that you didn't type it.
> SECCOPM_FILTER_APPLY
>
> SECCOMP_FILTER_SET, __NR_foo, "b=0"
> SECCOPM_FILTER_APPLY
>
> Will end up being:
>
> (foo: a == 0 || read: "1") && (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.
I think we pretty much agree although I'm pretty that we will have 1
filter per syscall. So the rules would really be (in your syntax)
Rule1: (foo: a == 0 && b == 0)
OR
Rule2: (read: "1")
Although logically the same, it's not just one huge rule. I don't see
any need for any operation other than an &&. Before the first "set" you
can add new syscalls. After the first set you can only && onto existing
syscalls. 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.
I do have a question on some syntax proposed a while back. Given:
SECCOMP_FILTER_SET, __NR_foo, "a=0"
SECCOMP_FILTER_SET, __NR_foo, "b=0"
SECCOMP_FILTER_APPLY
I would think to keep the interface consistent that should result in
foo: (a=0) && (b=0)
But I think the proposal was that we should instead have just
foo: (b=0)
What's the logic behind having a second call overwrite uncommitted
changes? I sorta feel like if I put it in there, I must have wanted it
in there :)
-Eric
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