Re: [PATCH 1/3] tpm: protect against locality counter underflow

[James Bottomley]

On Tue, 2024-02-20 at 22:31 +0000, Jarkko Sakkinen wrote:
> 
> 2. Because localities are not too useful these days given TPM2's
>    policy mechanism

Localitites are useful to the TPM2 policy mechanism.  When we get key
policy in the kernel it will give us a way to create TPM wrapped keys
that can only be unwrapped in the kernel if we run the kernel in a
different locality from userspace (I already have demo patches doing
this).

>  I cannot recall out of top of my head can
>    you have two localities open at same time.

I think there's a misunderstanding about what localities are: they're
effectively an additional platform supplied tag to a command.  Each
command can therefore have one and only one locality.  The TPM doesn't
have a concept of which locality is open; if you look at the reference
implementation, the simulator has a __plat__LocalitySet() function
which places all commands in the just set locality until you change to
a different one.

However, since the way localities are implemented (i.e. what triggers
_plat__LocalitySet()) is implementation defined, each physical TPM
device has a different way of doing the set (for instance, for TIS
TPM's locality is a function of the port set used to address the TPM;
for CRB TPMs it can be an additional tag on the buffer for command
submission).   I think the locality request/relinquish was modelled
after some other HW, but I don't know what.

James