Re: [RFC PATCH v1 3/4] tsm: Allow for mapping RTMRs to TCG TPM PCRs

[Kuppuswamy Sathyanarayanan]

On 1/21/24 11:46 PM, Samuel Ortiz wrote:
> On Sun, Jan 21, 2024 at 06:09:19PM -0800, Qinkun Bao wrote:
>>
>>> On Jan 21, 2024, at 8:31 AM, Samuel Ortiz <sameo@xxxxxxxxxxxx> wrote:
>>>
>>> On Tue, Jan 16, 2024 at 07:35:30PM -0800, Kuppuswamy Sathyanarayanan wrote:
>>>> On 1/16/24 5:24 PM, Dan Williams wrote:
>>>>> Kuppuswamy Sathyanarayanan wrote:
>>>>>> On 1/14/24 2:35 PM, Samuel Ortiz wrote:
>>>>>>> Many user space and internal kernel subsystems (e.g. the Linux IMA)
>>>>>>> expect a Root of Trust for Storage (RTS) that allows for extending
>>>>>>> and reading measurement registers that are compatible with the TCG TPM
>>>>>>> PCRs layout, e.g. a TPM. In order to allow those components to
>>>>>>> alternatively use a platform TSM as their RTS, a TVM could map the
>>>>>>> available RTMRs to one or more TCG TPM PCRs. Once configured, those PCR
>>>>>>> to RTMR mappings give the kernel TSM layer all the necessary information
>>>>>>> to be a RTS for e.g. the Linux IMA or any other components that expects
>>>>>>> a TCG compliant TPM PCRs layout.
>>>>>>>
>>>>>>> TPM PCR mappings are configured through configfs:
>>>>>>>
>>>>>>> // Create and configure 2 RTMRs
>>>>>>> mkdir /sys/kernel/config/tsm/rtmrs/rtmr0
>>>>>>> mkdir /sys/kernel/config/tsm/rtmrs/rtmr1
>>>>>>> echo 0 > /sys/kernel/config/tsm/rtmrs/rtmr0/index
>>>>>>> echo 1 > /sys/kernel/config/tsm/rtmrs/rtmr1/index
>>>>>>>
>>>>>>> // Map RTMR 0 to PCRs 4, 5, 6, 7 and 8
>>>>>>> echo 4-8 > /sys/kernel/config/tsm/rtmrs/rtmr0/tcg_map
>>>>>>>
>>>>>>> // Map RTMR 1 to PCRs 16, 17 and 18
>>>>>>> echo 16-18 > /sys/kernel/config/tsm/rtmrs/rtmr1/tcg_map
>>>>>> Any information on how this mapping will be used by TPM or IMA ?
>>>>>>
>>>>>> RTMR to PCR mapping is fixed by design, right? If yes, why allow
>>>>>> user to configure it. We can let vendor drivers to configure it, right?
>>>>> I assume the "vendor driver", that publishes the RTMR to the tsm-core,
>>>>> has no idea whether they will be used for PCR emulation, or not. The TPM
>>>>> proxy layer sitting on top of this would know the mapping of which RTMRs
>>>>> are recording a transcript of which PCR extend events.
>>>> My thinking is, since this mapping is ARCH-specific information
>>>> and fixed by design, it makes more sense to hide this detail in the
>>>> vendor driver than letting userspace configure it. If we allow users to
>>>> configure it, there is a chance for incorrect mapping.
>>> I think I agree with the fact that letting users configure that mapping
>>> may be error prone. But I'm not sure this is an architecture specific
>>> mapping, but rather a platform specific one. I'd expect the guest firmware
>>> to provide it through e.g. the MapPcrToMrIndex EFI CC protocol.
>>>
>>> So I agree I should remove the user interface for setting that mapping,
>>> and pass it from the provider capabilities instead. It is then up to the
>>> provider to choose how it'd build that information (hard coded, from
>>> EFI, etc).
>> The UEFI specification has defined the mapping relationship between the 
>> TDX RTMR and TPM PCRs (See https://uefi.org/specs/UEFI/2.10/38_Confidential_Computing.html#intel-trust-domain-extension). The current RTMR implementation in the boot loader 
>> is “hooked” in the implementation for the TPM. 
>>
>> When the bootloader needs to extend the PCR value, it calls 
>> `map_pcr_to_mr_index`  to retrieve the corresponding RTMR index and 
>> then extends the RTMR. Considering this behavior, I don’t think we should
>>  allow users to configure the mappings between the PCR and RTMR. (See https://github.com/rhboot/shim/pull/485/files <https://github.com/rhboot/shim/pull/485/files>).
> Just to be clear: I agree with that and I am going to send a v2 with
> that user interface removed.
> However I believe that we still need the TSM framework to know about
> these mappings and the question is where does the kernel get it from?
>
> You're suggesting that for TDX these mappings are architecturally
> defined, as described by the UEFI spec. For other architectures (CCA,
> CoVE) they are not (yet), so I'm suggesting to leave each TSM provider
> backend decide how the PCR to RTMR mapping should be built/fetched and
> provide it to the TSM framework through the tsm_capabilities structure
> that this patchset introduces. The TDX implementation could decide to
> hardcode it to the UEFI specification.

Agree. Lets leave it to TSM vendor drivers to provide this
mapping.


> Cheers,
> Samuel.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer