netdev - Re: [PATCH 2/3] Security: Implement disablenetwork semantics. (v4)

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Message-ID: <20100114150039.GB6322@us.ibm.com>
Date:	Thu, 14 Jan 2010 09:00:39 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Michael Stone <michael@...top.org>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, linux-security-module@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>, David Lang <david@...g.hm>,
	Oliver Hartkopp <socketcan@...tkopp.net>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Bryan Donlan <bdonlan@...il.com>,
	Evgeniy Polyakov <zbr@...emap.net>,
	"C. Scott Ananian" <cscott@...ott.net>,
	James Morris <jmorris@...ei.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Bernie Innocenti <bernie@...ewiz.org>,
	Mark Seaborn <mrs@...hic-beasts.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Am?rico Wang <xiyou.wangcong@...il.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Samir Bellabes <sam@...ack.fr>,
	Casey Schaufler <casey@...aufler-ca.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Kyle Moffett <kyle@...fetthome.net>
Subject: Re: [PATCH 2/3] Security: Implement disablenetwork semantics. (v4)

Quoting Pavel Machek (pavel@....cz):
> > Quoting Michael Stone (michael@...top.org):
> > > Serge Hallyn wrote:
> > > >Michael, I'm sorry, I should go back and search the thread for the
> > > >answer, but don't have time right now - do you really need
> > > >disablenetwork to be available to unprivileged users?
> > > 
> > > Rainbow can only drop the networking privileges when we know at app launch time
> > > (e.g. based on a manifest or from the human operator) that privileges can be
> > > dropped. Unfortunately, most of the really interesting uses of disablenetwork
> > > happen *after* rainbow has dropped privilege and handed control the app.
> > > Therefore, having an API which can be used by at least some low-privilege
> > > processes is important to me.
> > > 
> > > >is it ok to require CAP_SETPCAP (same thing required for dropping privs from
> > > >bounding set)?
> > > 
> > > Let me try to restate your idea:
> > > 
> > >   We can make disablenetwork safer by permitting its use only where explicitly
> > >   permitted by some previously privileged ancestor. The securebits facility
> > >   described in
> > > 
> > >     http://lwn.net/Articles/280279/
> > > 
> > >   may be a good framework in which to implement this control.
> > > 
> > > Did I understand correctly? If so, then yes, this approach seems like it would
> > > work for me.
> > 
> > That is a little more than I was saying this time though I think I
> > suggested it earlier.
> > 
> > But really I don't think anyone would care to separate a system into
> > some processes allowed to do unprivileged disablenetwork and other
> > processes not allowed to, so a (root-owned mode 644) sysctl seems just
> > as useful.
> 
> Global solution like that is always wrong. (And we have better
> solution available.)

All right, so Michael suggested securebits, I personally feel prctl would
be more appropriate, but in any case the suggestion then is:

foo_enable_disablenet() is either prctl(PR_ALLOW_DISABLENET) or
prctl(PR_SET_SECUREBITS, (1 << PR_ALLOW_DISABLENET) | (1 << PR_ALLOW_DISABLENET_LOCK))
and it requires privilege (CAP_NET_ADMIN presumably) to make this call.

prctl(PR_SET_DISABLENETWORK), or whatever Michael was using, does not
require privilege, but requires that foo_enable_disablenet() have been
previously called by a privileged app.

-serge
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