linux-kernel - Re: [PATCH v9 06/19] x86: Add early SHA-1 support for Secure Launch early measurements

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Message-ID: <550d15cd-5c48-4c20-92c2-f09a7e30adc9@citrix.com>
Date: Fri, 16 Aug 2024 12:01:15 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Thomas Gleixner <tglx@...utronix.de>,
 "Daniel P. Smith" <dpsmith@...rtussolutions.com>,
 "Eric W. Biederman" <ebiederm@...ssion.com>,
 Eric Biggers <ebiggers@...nel.org>
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, kexec@...ts.infradead.org,
 linux-efi@...r.kernel.org, iommu@...ts.linux-foundation.org,
 mingo@...hat.com, bp@...en8.de, hpa@...or.com, dave.hansen@...ux.intel.com,
 ardb@...nel.org, mjg59@...f.ucam.org, James.Bottomley@...senpartnership.com,
 peterhuewe@....de, jarkko@...nel.org, jgg@...pe.ca, luto@...capital.net,
 nivedita@...m.mit.edu, herbert@...dor.apana.org.au, davem@...emloft.net,
 corbet@....net, dwmw2@...radead.org, baolu.lu@...ux.intel.com,
 kanth.ghatraju@...cle.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v9 06/19] x86: Add early SHA-1 support for Secure Launch
 early measurements

On 15/08/2024 8:10 pm, Thomas Gleixner wrote:
> On Thu, Aug 15 2024 at 13:38, Daniel P. Smith wrote:
>> On 5/31/24 09:54, Eric W. Biederman wrote:
>>> Eric Biggers <ebiggers@...nel.org> writes:
>>>> That paragraph is also phrased as a hypothetical, "Even if we'd prefer to use
>>>> SHA-256-only".  That implies that you do not, in fact, prefer SHA-256 only.  Is
>>>> that the case?  Sure, maybe there are situations where you *have* to use SHA-1,
>>>> but why would you not at least *prefer* SHA-256?
>>> Yes.  Please prefer to use SHA-256.
>>>
>>> Have you considered implementing I think it is SHA1-DC (as git has) that
>>> is compatible with SHA1 but blocks the known class of attacks where
>>> sha1 is actively broken at this point?
>> We are using the kernel's implementation, addressing what the kernel 
>> provides is beyond our efforts. Perhaps someone who is interested in 
>> improving the kernel's SHA1 could submit a patch implementing/replacing 
>> it with SHA1-DC, as I am sure the maintainers would welcome the help.
> Well, someone who is interested to get his "secure" code merged should
> have a vested interested to have a non-broken SHA1 implementation if
> there is a sensible requirement to use SHA1 in that new "secure" code,
> no?

No.

The use of SHA-1 is necessary even on modern systems to avoid a
vulnerability.

It is the platform, not Linux, which decides which TPM PCR banks are active.

Linux *must* have an algorithm for every active bank (which is the
platform's choice), even if the single thing it intends to do is cap the
bank and use better ones.

Capping a bank requires updating the TPM Log without corrupting it,
which requires a hash calculation of the correct type for the bank.

~Andrew