Messages in this thread | | | Date | Wed, 3 Apr 2024 21:55:41 -0700 | Subject | Re: [PATCH v8 06/15] x86: Add early SHA support for Secure Launch early measurements | From | ross.philipson@oracle ... |
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On 4/3/24 4:56 PM, Eric Biggers wrote: > On Wed, Apr 03, 2024 at 09:32:02AM -0700, Andy Lutomirski wrote: >> On Fri, Feb 23, 2024, at 10:30 AM, Eric Biggers wrote: >>> On Fri, Feb 23, 2024 at 06:20:27PM +0000, Andrew Cooper wrote: >>>> On 23/02/2024 5:54 pm, Eric Biggers wrote: >>>>> On Fri, Feb 23, 2024 at 04:42:11PM +0000, Andrew Cooper wrote: >>>>>> Yes, and I agree. We're not looking to try and force this in with >>>>>> underhand tactics. >>>>>> >>>>>> But a blind "nack to any SHA-1" is similarly damaging in the opposite >>>>>> direction. >>>>>> >>>>> Well, reviewers have said they'd prefer that SHA-1 not be included and given >>>>> some thoughtful reasons for that. But also they've given suggestions on how to >>>>> make the SHA-1 support more palatable, such as splitting it into a separate >>>>> patch and giving it a proper justification. >>>>> >>>>> All suggestions have been ignored. >>>> >>>> The public record demonstrates otherwise. >>>> >>>> But are you saying that you'd be happy if the commit message read >>>> something more like: >>>> >>>> ---8<--- >>>> For better or worse, Secure Launch needs SHA-1 and SHA-256. >>>> >>>> The choice of hashes used lie with the platform firmware, not with >>>> software, and is often outside of the users control. >>>> >>>> Even if we'd prefer to use SHA-256-only, if firmware elected to start us >>>> with the SHA-1 and SHA-256 backs active, we still need SHA-1 to parse >>>> the TPM event log thus far, and deliberately cap the SHA-1 PCRs in order >>>> to safely use SHA-256 for everything else. >>>> --- >>> >>> Please take some time to read through the comments that reviewers have left on >>> previous versions of the patchset. >> >> So I went and read through the old comments, and I'm lost. In brief summary: >> >> If the hardware+firmware only supports SHA-1, then some reviewers would prefer >> Linux not to support DRTM. I personally think this is a bit silly, but it's >> not entirely unreasonable. Maybe it should be a config option? >> >> If the hardware+firmware does support SHA-256, then it sounds (to me, reading >> this -- I haven't dug into the right spec pages) that, for optimal security, >> something still needs to effectively turn SHA-1 *off* at runtime by capping >> the event log properly. And that requires computing a SHA-1 hash. And, to be >> clear, (a) this is only on systems that already support SHA-256 and that we >> should support and (b) *not* doing so leaves us potentially more vulnerable to >> SHA-1 attacks than doing so. And no SHA-256-supporting tooling will actually >> be compromised by a SHA-1 compromise if we cap the event log. >> >> So is there a way forward? Just saying "read through the comments" seems like >> a dead end. >> > > It seems there may be a justification for some form of SHA-1 support in this > feature. As I've said, the problem is that it's not explained in the patchset > itself. Rather, it just talks about "SHA" and pretends like SHA-1 and SHA-2 are > basically the same. In fact, SHA-1 differs drastically from SHA-2 in terms of > security. SHA-1 support should be added in a separate patch, with a clearly > explained rationale *in the patch itself* for the SHA-1 support *specifically*.
For the record, we were never trying to "pretend" or obfuscate the use of SHA-1. It was simply expedient to put the hash algorithm changes in one patch. We have now separated the patches for clarity and will add any text that explains our use and/or explain the issues with its use.
We went back through the comments and tried to address everything that came up about the use of SHA-1. We will review it all again before posting another patch set.
Thank you for your feedback. Ross
> > - Eric
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