linux-kernel - RE: [PATCH] intel_txt: add s3 userspace memory integrity verification

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Date:	Fri, 4 Dec 2009 12:17:42 -0800
From:	"Cihula, Joseph" <joseph.cihula@...el.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Pavel Machek <pavel@....cz>, "Wang, Shane" <shane.wang@...el.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"chrisw@...s-sol.org" <chrisw@...s-sol.org>,
	"jmorris@...ei.org" <jmorris@...ei.org>,
	"jbeulich@...ell.com" <jbeulich@...ell.com>,
	"peterm@...hat.com" <peterm@...hat.com>
Subject: RE: [PATCH] intel_txt: add s3 userspace memory integrity
	verification

> From: Andi Kleen [mailto:andi@...stfloor.org]
> Sent: Friday, December 04, 2009 12:10 PM
>
> On Fri, Dec 04, 2009 at 09:41:24AM -0800, Cihula, Joseph wrote:
> > > From: Andi Kleen [mailto:andi@...stfloor.org]
> > > Sent: Friday, December 04, 2009 9:14 AM
> > >
> > > > "bad stuff" would be the execution of any code (or use of any data that affects
> execution)
> > > that was not verified by tboot.  As long as panic() is within the code ranges MAC'ed by
> tboot
> > > (see above), it would be covered.  Do you know of some panic() code paths that are outside
> of
> > > this?
> > >
> > > Not code path, but the code called by panic (console drivers, debuggers etc.)
> > > can well use data that is stored >4GB
> > >
> > > This can include structures with indirect pointers, like notifier chains.
> > >
> > > Notifier chains have a special checker than can check
> > > for <4GB, but there are other call vectors too.
> >
> > Since, as you pointed out in a previous email, it is doubtful that there will be any user-
> visible output at this point, we can change this path to a tboot reset (which will give us
> some serial output at least).  Is it going to be similarly unsafe to do a printk()?
>
> Yes printk is similarly unsafe. It calls all the console machinery,
> which has a lot of data.
>
> Perhaps early_printk(), that is relatively self contained, but doesn't
> always work.
>
> Of course you would need to have a timeout before reset, and at this point the
> delay loops are not calibrated yet, so you don't know how to wait.

I would expect that early_printk() coupled with tboot's serial output would be sufficient for a case such as this.  If we've done our work correctly, loss of integrity should only occur when the system is attacked across the S3 transition--which should be fairly rare and which should place a premium on prevention of the attacked code from executing.  Esp. on servers, there may not be anyone to see console output anyway.  Does early_printk() and a tboot reset seem like a reasonable approach?

>
> -Andi
>
> --
> ak@...ux.intel.com -- Speaking for myself only.
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