linux-kernel - Re: [PATCH v6 02/14] Documentation/x86: Secure Launch kernel documentation

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Message-ID: <20230616165415.GA28537@srcf.ucam.org>
Date:   Fri, 16 Jun 2023 17:54:15 +0100
From:   Matthew Garrett <mjg59@...f.ucam.org>
To:     "Daniel P. Smith" <dpsmith@...rtussolutions.com>
Cc:     Ross Philipson <ross.philipson@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-crypto@...r.kernel.org, iommu@...ts.linux-foundation.org,
        kexec@...ts.infradead.org, linux-efi@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        ardb@...nel.org, James.Bottomley@...senpartnership.com,
        luto@...capital.net, nivedita@...m.mit.edu,
        kanth.ghatraju@...cle.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v6 02/14] Documentation/x86: Secure Launch kernel
 documentation

On Fri, Jun 16, 2023 at 12:44:27PM -0400, Daniel P. Smith wrote:
> 
> On 5/12/23 06:47, Matthew Garrett wrote:
> > On Thu, May 04, 2023 at 02:50:11PM +0000, Ross Philipson wrote:
> > > +Secure Launch does not interoperate with KASLR. If possible, the MLE should be
> > > +built with KASLR disabled::
> > 
> > Why does Secure Launch not interoperate with KASLR?
> > 
> > Re: IOMMUs
> 
> Until the IOMMU driver comes online, memory is protected by the PMRs regions
> requested by the Preamble (pre-launch code) in accordance with Intel TXT
> specifications and configured by the ACM. The KASLR randomizer will run
> before the IOMMU driver is able to come online and ensure frames used by the
> kernel are protected as well as frames that a driver may registered in a BAR
> are not blocked.

This seems unfortunate. Presumably we're not able to modify the PMRs at 
this point? This also seems like a potential issue for IOMMU config in 
general - the presumption is that the firmware should be configuring the 
IOMMU in such a way that DMA-capable devices can't attack the firmware 
while we're in the boot environment, and if KASLR is leaving a window 
there then it seems like we'd need to fix that?
 
> > > +It is recommended that no other command line options should be set to override
> > > +the defaults above.
> > 
> > What happens if they are? Does doing so change the security posture of
> > the system? If so, will the measurements be different in a way that
> > demonstrates the system is in an insecure state?
> > 
> 
> In an early version of the patch series this was enforced when turning on
> Secure Launch, but concerns were raised over this approach and was asked to
> allow the user to be able to shoot themselves in the foot. Overriding these
> values could render either an insecure state and/or an unstable system.

If we're in an insecure state, is that something that would show up in 
the form of different measurements?