linux-kernel - Re: [PATCH 01/12] Security: Add CAP_COMPROMISE

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Date:	Thu, 21 Mar 2013 10:37:25 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Matthew Garrett <matthew.garrett@...ula.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	James Morris <jmorris@...ei.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-security-module@...r.kernel.org" 
	<linux-security-module@...r.kernel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH 01/12] Security: Add CAP_COMPROMISE_KERNEL

Quoting Vivek Goyal (vgoyal@...hat.com):
...
> Giving CAP_MODIFY_KERNEL to processess upon signature verification
> will simplify things a bit.
> 
> Only thing is that signature verification alone is not sufficient. We
> also need to make sure after signature verification executable can
> not be modified in memory in any way. So that means atleast couple of
> things.

Also what about context?  If I construct a mounts namespace a certain
way, can I trick this executable into loading an old singed bzImage that
I had laying around?

ISTM the only sane thing to do, if you're going down this road,
is to have CAP_MODIFIY_KERNEL pulled from bounding set for everyone
except a getty started by init on ttyS0.  Then log in on serial
to update.  Or run a damon with CAP_MODIFIY_KERNEL which listens
to a init_net_ns netlink socket for very basic instructions, like
"find and install latest signed bzImage in /boot".  Then you can
at least trust that /boot for that daemon is not faked.
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