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

[Jarkko Sakkinen]

On Fri Feb 23, 2024 at 3:57 AM EET, Daniel P. Smith wrote:
> On 2/20/24 17:31, Jarkko Sakkinen wrote:
> > On Tue Feb 20, 2024 at 10:26 PM UTC, Jarkko Sakkinen wrote:
> >> On Tue Feb 20, 2024 at 8:54 PM UTC, Lino Sanfilippo wrote:
> >>> for (i = 0; i <= MAX_LOCALITY; i++)
> >>> 	__tpm_tis_relinquish_locality(priv, i);
> >>
> >> I'm pretty unfamiliar with Intel TXT so asking a dummy question:
> >> if Intel TXT uses locality 2 I suppose we should not try to
> >> relinquish it, or?
> >>
> >> AFAIK, we don't have a symbol called MAX_LOCALITY.
> > 
> > OK it was called TPM_MAX_LOCALITY :-) I had the patch set applied
> > in one branch but looked up with wrong symbol name.
> > 
> > So I reformalize my question to two parts:
> > 
> > 1. Why does TXT leave locality 2 open in the first place? I did
> >     not see explanation. Isn't this a bug in TXT?
>
> It does so because that is what the TCG D-RTM specification requires. 
> See Section 5.3.4.10 of the TCG D-RTM specification[1], the first 
> requirement is, "The DLME SHALL receive control with access to Locality 2"

>From below also the locality enumeration would be good to have
documented (as a reminder).

>
> > 2. Because localities are not too useful these days given TPM2's
> >     policy mechanism I cannot recall out of top of my head can
> >     you have two localities open at same time. So what kind of
> >     conflict happens when you try to open locality 0 and have
> >     locality 2 open?
>
> I would disagree and would call your attention to the TCG's 
> definition/motivation for localities, Section 3.2 of Client PTP 
> specification[2].
>
> "“Locality” is an assertion to the TPM that a command’s source is 
> associated with a particular component. Locality can be thought of as a 
> hardware-based authorization. The TPM is not actually aware of the 
> nature of the relationship between the locality and the component. The 
> ability to reset and extend notwithstanding, it is important to note 
> that, from a PCR “usage” perspective, there is no hierarchical 
> relationship between different localities. The TPM simply enforces 
> locality restrictions on TPM assets (such as PCR or SEALed blobs)."
>
> As stated, from the TPM specification perspective, it is not aware of 
> this mapping to components and leaves it to the platform to enforce.

Yeah, TPM is a passive component, not active actor, in everything.

The way I see locality as way to separate e.g. kernel and user space
driver TPM transactions is pretty much like actor-dependent salt
(e.g. if 0 was for user space and 1 was for kernel).

>
> "The protection and separation of the localities (and therefore the 
> association with the associated components) is entirely the 
> responsibility of the platform components. Platform components, 
> including the OS, may provide the separation of localities using 
> protection mechanisms such as virtual memory or paging."
>
> The x86 manufactures opted to adopt the D-RTM specification which 
> defines the components as follows:
>
> Locality 4: Usually associated with the CPU executing microcode. This is 
> used to establish the Dynamic RTM.
> Locality 3: Auxiliary components. Use of this is optional and, if used, 
> it is implementation dependent.
> Locality 2: Dynamically Launched OS (Dynamic OS) “runtime” environment.
> Locality 1: An environment for use by the Dynamic OS.
> Locality 0: The Static RTM, its chain of trust and its environment.
>
> And the means to protect and separate those localities are encoded in 
> the x86 chipset, i.e A D-RTM Event must be used to access any of the 
> D-RTM Localities (Locality1 - Locality4).
>
> For Intel, Locality 4 can only be accessed when a dedicated signal 
> between the CPU and the chipset is raised, thus only allowing the CPU to 
> utilize Locality 4. The CPU will then close Locality 4, authenticate and 
> give control to the ACM with access to Locality 3. When the ACM is 
> complete, it will instruct the chipset to lock Locality 3 and give 
> control to the DLME (MLE in Intel parlance) with Locality 2 open. It is 
> up to the DLME, the Linux kernel in this case, to decide how to assign 
> components to Locality 1 and 2.
>
> As to proposals to utilize localities by the Linux kernel, the only one 
> I was aware of was dropped because they couldn't open the higher localities.
>
> I would also highlight that the D-RTM implementation guide for Arm 
> allows for a hardware D-RTM event, which the vendor may choose to 
> implement a hardware/CPU enforced access to TPM localities. Thus, the 
> ability to support localities will also become a requirement for certain 
> Arm CPUs.
>
> [1] 
> https://trustedcomputinggroup.org/wp-content/uploads/TCG_D-RTM_Architecture_v1-0_Published_06172013.pdf
> [2] 
> https://trustedcomputinggroup.org/wp-content/uploads/PC-Client-Specific-Platform-TPM-Profile-for-TPM-2p0-v1p05p_r14_pub.pdf

BR, Jarkko