On 02/13/2018 02:59 PM, Laszlo Ersek wrote:
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).

ACPI code writing into fw_cfg sounds difficult.
I initially had PPI SeaBIOS code write into the TPM TIS device's memory into some custom addresses. I'd consider this a hack. Now we have that virtual memory device with those 0x400 bytes...

In these 0x400 bytes we have 256 bytes that are used for configuration flags describing the supported opcode as you previously described. This array allows us to decouple the firmware implementation from the ACPI code and we need not hard code what is supported in the firmware inside the ACPI code (which would be difficult to do anyway since in QEMU we would not what firmware will be started and what PPI opcodes are support) and the ppi sysfs entries in Linux for example show exactly those PPI opcodes that are supported. The firmware needs to set those flags and the firmware knows what it supports.

I hope we can settle that this device is the right path.


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

I experimented with what Igor had suggested with the fw_cfg file callback and so on.

- QEMU is free to move the device or register block around, from release
   to release, without disturbing migration.

I think we should basically limit the firmware to writing two addresses into this fw_cfg file: - SeaBIOS: write back the same address that QEMU suggested in the file (currently 0xfed4 5000)
- EDK2: write back 0xffff 0000

No other address would be accepted by QEMU since presumably QEMU knows where otherwise address collisions can occur.


Thanks!
Laszlo



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