Re: [PATCH] ima: select the SHA384 PCR bank for the boot aggregate

[Singh, Jashandeep]

Thanks, Mimi and Roberto.

I will explore this approach and share the updated patch.

Regards,
Jashan

> On 27 Aug 2026, at 9:19 PM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> 
> On Thu, 2026-08-27 at 16:56 +0200, Roberto Sassu wrote:
>> On Thu, 2026-08-27 at 14:46 +0000, Singh, Jashandeep wrote:
>>> Mimi, the patch covers the case where the TPM is provisioned with a single PCR
>>> bank, SHA-384, while the default IMA hash is configured as SHA-256.
>>> 
>>> The SHA-384 bank is present in nr_allocated_banks, but it matches neither the
>>> configured default (SHA-256) nor the current hardcoded fallbacks (SHA-256, then
>>> SHA-1). As a result, bank_idx remains -1, and we hit "No suitable TPM algorithm
>>> for boot aggregate", leaving the boot aggregate zeroed.
>>> 
>>> Roberto, it is not strictly true that the boot aggregate algorithm is always the
>>> same as the default IMA hash algorithm.
>>> 
>>> When the bank corresponding to the default hash is not allocated, the existing
>>> SHA-256/SHA-1 fallback logic selects a different bank than the default.
>>> Therefore, the boot aggregate can already use a different algorithm from the
>>> configured IMA hash.
>>> 
>>> My patch adds SHA-384 as one more fallback for the case where SHA-384 is the
>>> only allocated bank. This allows users with such a configuration to have the
>>> boot aggregate computed, without requiring them to change their default IMA
>>> hash to SHA-384.
>> 
>> That would work for your use case, but what about anyone using a single
>> SHA-512 PCR bank?
>> 
>> Would it be fine to add a new Kconfig and kernel option to specify a
>> custom boot aggregate algorithm?
> 
> Yes, I'm fine with defining a new Kconfig and kernel option.  
> 
> Examples:
> sha384: <digest> boot_aggregate, where the digest is calculated based on the TPM
> 384 bank PCRs.
> 
> sha512: <digest> boot_aggregate, where the digest is calculated based on the TPM
> 512 bank PCRs.
> 
> Mimi