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*. - Eric
