Re: [PATCH v6 06/14] x86: Add early SHA support for Secure Launch early measurements

[Eric Biggers]

On Fri, May 12, 2023 at 01:24:22PM +0100, Andrew Cooper wrote:
> On 12/05/2023 12:58 pm, Ard Biesheuvel wrote:
> > On Fri, 12 May 2023 at 13:28, Matthew Garrett <mjg59@xxxxxxxxxxxxx> wrote:
> >> On Fri, May 12, 2023 at 01:18:45PM +0200, Ard Biesheuvel wrote:
> >>> On Fri, 12 May 2023 at 13:04, Matthew Garrett <mjg59@xxxxxxxxxxxxx> wrote:
> >>>> On Tue, May 09, 2023 at 06:21:44PM -0700, Eric Biggers wrote:
> >>>>
> >>>>> SHA-1 is insecure.  Why are you still using SHA-1?  Don't TPMs support SHA-2
> >>>>> now?
> >>>> TXT is supported on some TPM 1.2 systems as well. TPM 2 systems are also
> >>>> at the whim of the firmware in terms of whether the SHA-2 banks are
> >>>> enabled. But even if the SHA-2 banks are enabled, if you suddenly stop
> >>>> extending the SHA-1 banks, a malicious actor can later turn up and
> >>>> extend whatever they want into them and present a SHA-1-only
> >>>> attestation. Ideally whatever is handling that attestation should know
> >>>> whether or not to expect an attestation with SHA-2, but the easiest way
> >>>> to maintain security is to always extend all banks.
> >>>>
> >>> Wouldn't it make more sense to measure some terminating event into the
> >>> SHA-1 banks instead?
> >> Unless we assert that SHA-1 events are unsupported, it seems a bit odd
> >> to force a policy on people who have both banks enabled. People with
> >> mixed fleets are potentially going to be dealing with SHA-1 measurements
> >> for a while yet, and while there's obviously a security benefit in using
> >> SHA-2 instead it'd be irritating to have to maintain two attestation
> >> policies.
> > I understand why that matters from an operational perspective.
> >
> > However, we are dealing with brand new code being proposed for Linux
> > mainline, and so this is our only chance to push back on this, as
> > otherwise, we will have to maintain it for a very long time.
> >
> > IOW, D-RTM does not exist today in Linux, and it is up to us to define
> > what it will look like. From that perspective, it is downright
> > preposterous to even consider supporting SHA-1, given that SHA-1 by
> > itself gives none of the guarantees that D-RTM aims to provide. If
> > reducing your TCB is important enough to warrant switching to this
> > implementation of D-RTM, surely you can upgrade your attestation
> > policies as well.
> 
> You're suggesting that because Linux has been slow to take D-RTM over
> the past decade, you're going to intentionally break people with older
> hardware just because you don't feel like using an older algorithm?
> 
> That's about the worst possible reason to not take support.
> 
> There really are people in the world with older TPM 1.2 systems where
> this D-RTM using SHA1 only is an improvement over using the incumbent tboot.
> 
> ~Andrew

This patchset is proposing a new kernel feature.  So by definition, there are no
existing users of it that can be broken.

The fact is, SHA-1 is cryptographically broken.  It isn't actually about how
"old" the algorithm is, or what anyone's "feelings" are.

Maybe a renaming from Secure Launch to simply Launch is in order?

- Eric