Message-ID: <20091230201712.GA23999@us.ibm.com>
Date:	Wed, 30 Dec 2009 14:17:12 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Andrew G. Morgan" <morgan@...nel.org>,
	Bryan Donlan <bdonlan@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Benny Amorsen <benny+usenet@...rsen.dk>,
	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>,
	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][PATCH v2] Unprivileged: Disable raising of privileges

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> "Serge E. Hallyn" <serue@...ibm.com> writes:
> 
> > Quoting Andrew G. Morgan (morgan@...nel.org):
> >> Eric,
> >> 
> >> I'm not clear why capabilities need to be manipulated by this feature
> >> (the pure capability support already has a feature for disabling
> >> privilege and blocking unsafe, or insufficient privilege, execution).
> >
> > Not entirely - this option would also prevent file capabilities from
> > being honored.
> 
> All my patch does is verify the caller doesn't have privilege.

No, you shortcut security/commoncap.c:get_file_caps() if (bprm->nosuid),
which is set if test_tsk_thread_flag(current, TIF_NOSUID) at exec.

So if we're in this new no-suid mode, then file capabilities are not
honored.

Which is the right thing to do.

> >> Perhaps I'm just unclear what features can be more safely enabled with
> >> this in effect - that is, your description suggests that this is why
> >> you are doing this, but leaves it unclear what they are. Could you
> >> take a few moments to enumerate some of them?
> >
> > There are two desirable features which are at the moment unsafe for
> > unprivileged users, because it allows them to fool privileged (setuid
> > or bearing file capabilities) programs.  One is to unconditionally
> > restrict privilege to yourself and all your descendents.  The recent
> > disablenetwork patchset is one example.  The other is the ability to
> > make substantial changes to your environment in a private namespace.
> > A private namespace can protect already-running privileged program,
> > but cannot protect privilege-bearing binaries.  Unless we prevent
> > them from bearing privilege.  Which is what this patch does.
> 
> Effectively by ensuring privileges can not be raised this removes
> the set of circumstances that lead to the sendmail capabilities bug.
> 
> So any kernel feature that requires capabilities only because not
> doing so would break backwards compatibility with suid applications.
> This includes namespace manipulation, like plan 9.
> This includes unsharing pid and network and sysvipc namespaces.
> 
> There are probably other useful but currently root only features
> that this will allow to be used by unprivileged processes, that
> I am not aware of.
> 
> In addition to the fact that knowing privileges can not be escalated
> by a process is a good feature all by itself.  Run this in a chroot
> and the programs will never be able to gain root access even if
> there are suid binaries available for them to execute.
> 
> Eric
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