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

[Laszlo Ersek <lersek@redhat.com>]

On 02/13/18 20:37, Kevin O'Connor wrote:
> On Tue, Feb 13, 2018 at 05:16:49PM +0100, Laszlo Ersek wrote:
>> On 02/12/18 21:49, Stefan Berger wrote:
>>> On 02/12/2018 03:46 PM, Kevin O'Connor wrote:
>>>> 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.
>>
>> Here's what I suggest:
>>
>> Please everyone continue working on this, according to Kevin's &
>> Stefan's description, but focus on QEMU and SeaBIOS *only*. Ignore edk2
>> for now.
> 
> If this were targetted at SeaBIOS, I'd look for a simpler
> QEMU/firmware interface.  Something like:
> 
> A - QEMU produces AML code implementing the standard PPI ACPI
>     interface that generates a request code and stores it in the
>     device memory of an existing device (eg, writable fw_cfg or an
>     extension field in the existing emulated TPM device).
> 
> B - after a reboot the firmware extracts the PPI request code
>     (produced in step A) and performs the requested action (if the TPM
>     is configured to accept OS generated codes).
> 
> That is, skip steps 1 and 2 from the original proposal.

I think A/B can work fine, as long as
- the firmware can somehow dynamically recognize the device / "register
  block" that the request codes have to be pulled from, and
- QEMU is free to move the device or register block around, from release
  to release, without disturbing migration.

Thanks!
Laszlo