linux-kernel - Re: [RFC][PATCH v3] Unprivileged: Disable raising of privileges

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Message-ID: <m1aawzm1tc.fsf@fess.ebiederm.org>
Date:	Thu, 31 Dec 2009 10:32:47 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Andrew G. Morgan" <morgan@...nel.org>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Bryan Donlan <bdonlan@...il.com>,
	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 v3] Unprivileged: Disable raising of privileges

"Andrew G. Morgan" <morgan@...nel.org> writes:

> Why not implement this as another securebit? So far as I can see the
> whole thing can be implemented in the capability LSM.
>
> What is less clear to me is whether per-process 'disabling of setuid
> bits on files' should force mandatory disabling of file capabilities.
> It seems as if disabling the transition of one luser to another luser
> through a setuid executable is something distinct from privilege
> escalation.
>
> Since there is already independent support for disabling file
> capabilities (the privilege escalation part), I see these two
> mechanisms as separable.

The goal is to disable privilege escalation.

The anatomy of the sendmail capabilities bug as I understand it was:

- unprivileged process took action to prevent gaining a capability.
- exec'd suid sendmail.
- sendmail took action as root because it could not become someone else.

I would like to trivially stop that entire class of exploit by making
execing a suid ( or equivalent ) executable impossible.

Once that hole is closed we can enable things like chroot without
privilege.

If there is a way to express this with capabilities today I would be
more than happy to.

Eric
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