linux-kernel - Re: RFC: disablenetwork facility. (v4)

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Message-ID: <20091229212722.GA20178@us.ibm.com>
Date:	Tue, 29 Dec 2009 15:27:22 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Bryan Donlan <bdonlan@...il.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	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>,
	Evgeniy Polyakov <zbr@...emap.net>,
	"C. Scott Ananian" <cscott@...ott.net>,
	James Morris <jmorris@...ei.org>,
	Bernie Innocenti <bernie@...ewiz.org>,
	Mark Seaborn <mrs@...hic-beasts.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Américo Wang <xiyou.wangcong@...il.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Samir Bellabes <sam@...ack.fr>,
	Casey Schaufler <casey@...aufler-ca.com>,
	Pavel Machek <pavel@....cz>, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: RFC: disablenetwork facility. (v4)

Quoting Bryan Donlan (bdonlan@...il.com):
> On Tue, Dec 29, 2009 at 1:36 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
> > Bryan Donlan <bdonlan@...il.com> writes:
> >
> >> On Tue, Dec 29, 2009 at 11:39 AM, Serge E. Hallyn <serue@...ibm.com> wrote:
> >>> Quoting Bryan Donlan (bdonlan@...il.com):
> >>>> On Tue, Dec 29, 2009 at 10:11 AM, Serge E. Hallyn <serue@...ibm.com> wrote:
> >>>> > Eric, let me specifically point out a 'disable setuid-root'
> >>>> > problem on linux: root still owns most of the system even when
> >>>> > it's not privileged.  So does "disable setuid-root" mean
> >>>> > we don't allow exec of setuid-root binaries at all, or that
> >>>> > we don't setuid to root, or that we just don't raise privileges
> >>>> > for setuid-root?
> >>>>
> >>>> I, for one, think it would be best to handle it exactly like the
> >>>> nosuid mount option - that is, pretend the file doesn't have any
> >>>> setuid bits set. There's no reason to deny execution; if the process
> >>>> would otherwise be able to execute it, it can also copy the file to
> >>>> make a non-suid version and execute that instead. And some programs
> >>>> can operate with reduced function without setuid. For example, screen
> >>>> comes to mind; it needs root to share screen sessions between multiple
> >>>> users, but can operate for a single user just fine without root, and
> >>>> indeed the latter is usually the default configuration.
> >>>
> >>> That's fine with me, seems safe for a fully unprivileged program to
> >>> use, and would make sense to do through one of the securebits set
> >>> with prctl(PR_SET_SECUREBITS).
> >>>
> >>> In addition, I assume we would also refuse to honor file capabilities?
> >>
> >> Yes - essentially a one-time switch saying "never allow me to gain
> >> capabilities again".
> >
> > That is what I was thinking.  Does setresuid case problems?  Assuming
> > the application that drop permissions could have successfully
> > called setresuid?
> 
> It's probably reasonable to require that real == effective == saved ==
> fs UID (and same for GID); anything else brings up sticky issues of
> "which UID is a higher capability?"

I think i disagree.  A uid is just a uid (or should be).  One day we may
have a way for a factotum-style daemon to grant the ability to an unpriv
task to setuid without CAP_SETUID.  I think slingling uids and gids
around that you already have access to should be fine.

> If a process does this call, it's effectively saying that the only way
> it's going to be accessing resources beyond its current UID and
> capabilities is by talking to another process over a (unix domain)
> socket.
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