linux-kernel - Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to kernel-only use

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Message-ID: <Y9K2mOsmB1+CFk9l@kernel.org>
Date:   Thu, 26 Jan 2023 17:21:28 +0000
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     James Bottomley <jejb@...ux.ibm.com>
Cc:     William Roberts <bill.c.roberts@...il.com>,
        Matthew Garrett <mgarrett@...ora.tech>,
        Evan Green <evgreen@...omium.org>,
        linux-kernel@...r.kernel.org, corbet@....net,
        linux-integrity@...r.kernel.org,
        Eric Biggers <ebiggers@...nel.org>, gwendal@...omium.org,
        dianders@...omium.org, apronin@...omium.org,
        Pavel Machek <pavel@....cz>, Ben Boeckel <me@...boeckel.net>,
        rjw@...ysocki.net, Kees Cook <keescook@...omium.org>,
        dlunev@...gle.com, zohar@...ux.ibm.com, linux-pm@...r.kernel.org,
        Matthew Garrett <mjg59@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Peter Huewe <peterhuewe@....de>
Subject: Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to
 kernel-only use

On Tue, Jan 24, 2023 at 07:38:04AM -0500, James Bottomley wrote:
> On Mon, 2023-01-23 at 11:48 -0600, William Roberts wrote:
> > On Fri, Jan 20, 2023 at 9:29 PM Jarkko Sakkinen <jarkko@...nel.org>
> > wrote:
> > > 
> > > On Sat, Jan 14, 2023 at 09:55:37AM -0500, James Bottomley wrote:
> > > > On Tue, 2023-01-03 at 13:10 -0800, Matthew Garrett wrote:
> > > > > On Tue, Jan 3, 2023 at 1:05 PM William Roberts
> > > > > <bill.c.roberts@...il.com> wrote:
> > > > > 
> > > > > > What's the use case of using the creation data and ticket in
> > > > > > this context? Who gets the creationData and the ticket?
> > > > > > Could a user supplied outsideInfo work? IIRC I saw some
> > > > > > patches flying around where the sessions will get encrypted
> > > > > > and presumably correctly as well. This would allow the
> > > > > > transfer of that outsideInfo, like the NV Index PCR value to
> > > > > > be included and integrity protected by the session HMAC.
> > > > > 
> > > > > The goal is to ensure that the key was generated by the kernel.
> > > > > In the absence of the creation data, an attacker could generate
> > > > > a hibernation image using their own key and trick the kernel
> > > > > into resuming arbitrary code. We don't have any way to pass
> > > > > secret data from the hibernate kernel to the resume kernel, so
> > > > > I don't think there's any easy way to do it with outsideinfo.
> > > > 
> > > > Can we go back again to why you can't use locality?  It's exactly
> > > > designed for this since locality is part of creation data. 
> > > > Currently everything only uses locality 0, so it's impossible for
> > > > anyone on Linux to produce a key with anything other than 0 in
> > > > the creation data for locality.  However, the dynamic launch
> > > > people are proposing that the Kernel should use Locality 2 for
> > > > all its operations, which would allow you to distinguish a key
> > > > created by the kernel from one created by a user by locality.
> > > > 
> > > > I think the previous objection was that not all TPMs implement
> > > > locality, but then not all laptops have TPMs either, so if you
> > > > ever come across one which has a TPM but no locality, it's in a
> > > > very similar security boat to one which has no TPM.
> > > 
> > > Kernel could try to use locality 2 and use locality 0 as fallback.
> > 
> > I don't think that would work for Matthew, they need something
> > reliable to indicate key provenance.
> 
> No, I think it would be good enough: locality 0 means anyone (including
> the kernel on a machine which doesn't function correctly) could have
> created this key.  Locality 2 would mean only the kernel could have
> created this key.
> 
> By the time the kernel boots and before it loads the hibernation image
> it will know the answer to the question "does my TPM support locality
> 2", so it can use that in its security assessment: if the kernel
> supports locality 2 and the key wasn't created in locality 2 then
> assume an attack.  Obviously, if the kernel doesn't support locality 2
> then the hibernation resume has to accept any old key, but that's the
> same as the situation today.

This sounds otherwise great to me but why bother even allowing a 
machine with no-locality TPM to be involved with hibernate? Simply
detect locality support during driver initialization and disallow
sealed hibernation (or whatever the feature was called) if localities
were not detected.

I get supporting old hardware with old features but it does not make
sense to maintain new features with hardware, which clearly does not
scale, right?

BR, Jarkko