Re: [PATCH v5 0/5] Lazy flush for the auth session

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, 2024-10-11 at 18:10 +0200, Roberto Sassu wrote:
> Initially, I thought that maybe it would not be good to have an event
> log with unmodified and altered measurement entries. Then, I tried to
> think if we can really prevent an active interposer from injecting
> arbitrary PCR extends and pretending that those events actually
> happened.
> 
> If I understood James's cover letter correctly, the kernel can detect
> whether a TPM reset occurred, but not that a PCR extend occurred
> (maybe
> with a shadow PCR?).

We can detect TPM reset indirectly. I.e. null seed re-randomizes
per reset.

> 
> Second point, do we really want to take the responsibility to disable
> the protection on behalf of users? Maybe a better choice is to let
> them
> consciously disable HMAC protection.

So when IMA is not used already with these fixes we get good
results. And for tpm2_get_random() we can make the algorithm
smarter. All in all we have good path ongoing for "desktop
use case" that I would keep thing way there are or at least
postpone any major decisions just a bit.

For server/IMA use case I'll add a boot parameter it can be
either on or off by default, I will state that in the commit
message and we'll go from there.

> 
> So, maybe we should keep the HMAC protection enabled, and if the
> number
> of PCR extends is above a certain threshold, we can print a warning
> message in the kernel log.
> 
> Roberto

BR, Jarkko