Re: [Qemu-devel] [PATCH v2 4/4] acpi: build TPM Physical Presence interface

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 02/12/2018 03:46 PM, Kevin O'Connor wrote:
> On Fri, Feb 09, 2018 at 03:19:31PM -0500, Stefan Berger wrote:
>> The PPI device in this patch series allocates 0x400 bytes. 0x200 bytes are
>> used by the OperationRegion() in this patch series. The rest was thought of
>> for future extensions.
>>
>> To allow both firmwares to use PPI, we would need to be able to have the
>> OperationRegion() be flexible and located at 0xFFFF 0000 for EDK2 and 0xFED4
>> 5000 for SeaBIOS, per choice of the firmware. One way to achieve this would
>> be to have the firmwares choose their operation region base address by
>> writing into the PPI memory device at offset 0x200 (for example). A '1'
>> there would mean that we want the OperationRegion() at 0xFFFF 0000, and a
>> '2' would mean at the base address of the PPI device (0xFED4 5000). This
>> could be achieved by declaring a 2nd OperationRegion() in the ACPI code that
>> is located at 0xFED4 5200 and we declare that 1st OperationRegion()'s
>> address based on findings from there. Further, a flags variable at offset
>> 0x201 could indicate whether an SMI is needed to enter SMM or not.
>> Obviously, the ACPI code would become more complex to be able to support
>> both firmwares at the same time.
>>
>> This is a lot of details but I rather post this description before posting
>> more patches. To make these changes and get it to work with at least SeaBIOS
>> is probably fairly easy. But is this acceptable?
> I'm not sure I fully understand the goals of the PPI interface.
> Here's what I understand so far:
>
> The TPM specs define some actions that are considered privileged.  An
> example of this would be disabling the TPM itself.  In order to
> prevent an attacker from performing these actions without
> authorization, the TPM specs define a mechanism to assert "physical
> presence" before the privileged action can be done.  They do this by
> having the firmware present a menu during early boot that permits
> these privileged operations, and then the firmware locks the TPM chip
> so the actions can no longer be done by any software that runs after
> the firmware.  Thus "physical presence" is asserted by demonstrating
> one has console access to the machine during early boot.
>
> The PPI spec implements a work around for this - presumably some found
> the enforcement mechanism too onerous.  It allows the OS to provide a
> request code to the firmware, and on the next boot the firmware will
> take the requested action before it locks the chip.  Thus allowing the
> OS to indirectly perform the privileged action even after the chip has
> been locked.  Thus, the PPI system seems to be an "elaborate hack" to
> allow users to circumvent the physical presence mechanism (if they
> choose to).

Correct.
>
> Here's what I understand the proposed implementation involves:
>
> 1 - in addition to emulating the TPM device itself, QEMU will also
>      introduce a virtual memory device with 0x400 bytes.
Correct.
>
> 2 - on first boot the firmware (seabios and uefi) will populate the
>      memory region created in step 1.  In particular it will fill an
>      array with the list of request codes it supports.  (Each request
>      is an 8bit value, the array has 256 entries.)
Correct. Each firmware would fill out the 256 byte array depending on 
what it supports. The 8 bit values are basically flags and so on.
> 3 - QEMU will produce AML code implementing the standard PPI ACPI
>      interface.  This AML code will take the request, find the table
>      produced in step 1, compare it to the list of accepted requests
>      produced in step 2, and then place the 8bit request in another
>      qemu virtual memory device (at 0xFFFF0000 or 0xFED45000).

Correct.

Now EDK2 wants to store the code in a UEFI variable in NVRAM. We 
therefore would need to trigger an SMI. In SeaBIOS we wouldn't have to 
do this.

> 4 - the OS will signal a reboot, qemu will do its normal reboot logic,
>      and the firmware will be run again.
>
> 5 - the firmware will extract the code written in stage 3, and if the
>      tpm device has been configured to accept PPI codes from the OS, it
>      will invoke the requested action.

SeaBIOS would look into memory to find the code. EDK2 will read the code 
from a UEFI variable.

> Did I understand the above correctly?
I think so. With the fine differences between SeaBIOS and EDK2 pointed out.

> Separately, is there someone clamoring for PPI support today?  That
> is, is the goal to implement PPI to make QEMU more spec compliant, or
> is there someone struggling to perform a particular task today that
> this support would improve?

We could defer the implementation of this. My main goal was to to 
support TPM migration in QEMU and the PPI device also needs to be 
migrated as part of TPM migration. So that's why I am looking at PPI now.

     Stefan
> Thanks,
> -Kevin
>