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

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Message-ID: <m1k4w4633t.fsf@fess.ebiederm.org>
Date:	Wed, 30 Dec 2009 04:47:02 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Bryan Donlan <bdonlan@...il.com>
Cc:	"Serge E. Hallyn" <serue@...ibm.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] Unprivileged: Disable acquisition of privileges

Bryan Donlan <bdonlan@...il.com> writes:

> On Tue, Dec 29, 2009 at 11:33 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Bryan Donlan <bdonlan@...il.com> writes:
>>
>>> Is this sufficient for other security models such as selinux or
>>> TOMOYO? Can processes in these models gain privileges through means
>>> not restricted here?
>>
>> The LSM is primarily about returning -EPERM more often.
>> Except for the prctl and the capability hooks I am not aware
>> of anywhere a LSM can increase a processes capabilities.
>
> I'm more concerned about a case where a privilege that the LSM
> currently denies is lifted by execing some executable - this is still
> an increase in privilege, even though the LSM only adds additional
> restrictions. That is:
>
> 1) Initial state: LSM denies access to /somefile (although normal
> POSIX permissions would permit access)
> 2) Disable capability-gaining
> 3) Disable network access with proposed API
> 4) Exec some application, which is labeled in a way that permits
> access to /somefile
> 5) Application fails to access the network, then does something to /somefile
>
> I'm not entirely sure if step 4) can happen in any of the currently
> existing LSMs - if it's not possible to gain privileges in them via a
> suid-like mechanism, this isn't a problem, but it's something that
> needs to be checked for.

A reasonable concern.  When the glitches get worked out of this patch
I intend to allow much more dangerous things like unprivileged unsharing
of all of the namespaces, and unprivileged mounts.

It appears I missed a place where MNT_NOSUID was handled in selinux.
So I will be adding a bprm->nosuid field so I don't have to duplicate
the MNT_NOSUID check everywhere it is used.

I don't understand TOMOYO  I think it is file based access control,
which suggests there is not a suid like mechanism.

Smack and selinux are label based.  Selinux at least can switch labels
on exec, but it handles NOSUID already.

Looking a little farther if I assume that lsm implementations that
implement the set_creds hook need attention.  Only selinux has
an interesting set_creds implementation and it handles nosuid already.

So I think we are ok.

Eric
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