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

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Message-ID: <4F65016F6CB04E49BFFA15D4F7B798D9AEDDD552@orsmsx506.amr.corp.intel.com>
Date:	Fri, 4 Dec 2009 09:41:24 -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 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()?

> > > > > checksummed by tboot, attacker may be able to hijack code execution
> > > > > and bypass your protection, no?
> > > > Yes, kernel code is audited by tboot before resume.
> > >
> > > So no, you did not audit do_suspend_lowlevel to make sure it does not
> > > follow function pointers. Bad.
> >
> > We aren't aware of any code or data used by the resume path that is outside of the tboot-
> MAC'ed regions above--if you can point out any then we will gladly address them.
>
> Code coverage is not enough, you need data coverage too.  If someone
> modifies kernel data it's typically easy to subvert code as a next step.

Agreed, which is why I said "code or data".  We'll take another look at the couple of fns that are within this path, but if you have any specific examples can you please post them.

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