Re: [PATCH] intel_txt: add s3 userspace memory integrity verification

[Andi Kleen <andi@firstfloor.org>]

On Fri, Dec 04, 2009 at 09:41:24AM -0800, Cihula, Joseph wrote:
> > From: Andi Kleen [mailto:andi@firstfloor.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.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.