linux-kernel - Re: Kdump with signed images

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Date:	Fri, 26 Oct 2012 14:37:29 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Matthew Garrett <mjg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Khalid Aziz <khalid@...ehiking.org>, kexec@...ts.infradead.org,
	horms@...ge.net.au, Dave Young <dyoung@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	Roberto Sassu <roberto.sassu@...ito.it>,
	Kees Cook <keescook@...omium.org>
Subject: Re: Kdump with signed images

On Fri, 2012-10-26 at 13:06 -0400, Vivek Goyal wrote:
> On Fri, Oct 26, 2012 at 03:39:16AM +0100, Matthew Garrett wrote:
> > On Thu, Oct 25, 2012 at 09:15:58PM -0400, Mimi Zohar wrote:
> > 
> > > On a running system, the package installer, after verifying the package
> > > integrity, would install each file with the associated 'security.ima'
> > > extended attribute.  The 'security.evm' digital signature would be
> > > installed with an HMAC, calculated using a system unique key. 
> > 
> > The idea isn't to prevent /sbin/kexec from being modified after 
> > installation - it's to prevent it from being possible to install a 
> > system that has a modified /sbin/kexec. Leaving any part of this up to 
> > the package installer means that it doesn't solve the problem we're 
> > trying to solve here. It must be impossible for the kernel to launch any 
> > /sbin/kexec that hasn't been signed by a trusted key that's been built 
> > into the kernel, and it must be impossible for anything other than 
> > /sbin/kexec to make the kexec system call.
> 
> I am kind of lost now so just trying to summarize whatever I have
> learned so far from this thread.

Thanks for summarizing.

> - So say we can sign /sbin/kexec at build time and distros can do that.
> - Verify the signature at exec time using kernel keyring and if
>   verification happens successfully, say process gains extra capability.
> - Use this new capability to determine whether kexec_load() will be
>   successful or not.
> 
> Even if we can do all this, it still has the issue of being able to
> stop the process in user space and replace the code at run time
> and be able to launch unsigned kernel.
> 
> So until and unless we have a good solution to verify application's
> integrity/authneticity at the time of kexec_load() system call we
> still have the problem. And I don't think we have come up with a
> solution for that yet (until and unless I missed something).
>   
> Thanks
> Vivek
> 

Agreed, you need a new LSM/integrity hook.

thanks,

Mimi

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