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

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Date:	Thu, 21 Mar 2013 10:58:23 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	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>
Subject: Re: [PATCH 01/12] Security: Add CAP_COMPROMISE_KERNEL

Quoting Vivek Goyal (vgoyal@...hat.com):
> On Thu, Mar 21, 2013 at 10:37:25AM -0500, Serge E. Hallyn wrote:
> > 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?
> 
> We were thinking that /sbin/kexec will need to verify bzImage signature
> before loading it. 
> 
> Key for verification is in kernel so idea was to take kernel's help
> in verifying signature.
> 
> Not sure how exactly the interface should look like. 
> 
> - I was thinking may be process can mmap() the bzImage with MAP_LOCKED.
>   We can create additional flag say MAP_VERIFY_SIG_POST, which tries
>   to verify signature/integrity of mapped region of file after mapping and
>   locking pages and mmap() fails if signature verification fails.
> 
> There have been suggestions about creating new system call altogether.
> 
> > 
> > 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.
> 
> daemon does not have the key and can't verify signature of signed
> bzImage. Even if it had the key, it can't trust the crypto code for
> signature verification as none of that is signed.

I'm not saying not to use the kernel to verify the signature.

-serge
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