[SeaBIOS] Saving a few bytes across a reboot

Laszlo Ersek lersek at redhat.com
Wed Feb 7 15:18:46 CET 2018

On 02/07/18 14:51, Stefan Berger wrote:
> On 01/10/2018 08:22 AM, Laszlo Ersek wrote:
>> Stefan,
>> On 01/09/18 20:02, Stefan Berger wrote:
>>> Another twist is that Intel's EDK2 also implements this but the data
>>> structure layout is different and they use SMM + SMIs etc.
>>> https://github.com/tianocore/edk2/blob/master/SecurityPkg/Tcg/Tcg2Smm/Tpm.asl#L81
>> As I described in my investigation linked from
>> <https://bugzilla.tianocore.org/show_bug.cgi?id=594#c5>, we should not
>> include the Tcg2Smm driver in OVMF, for TPM enablement -- at least for
>> the short & mid terms.
>> What does the Tcg2Smm driver do? In section (2f), I described that the
>> driver installs two tables, "TPM2" and an "SSDT".
>> - The TPM2 table from this driver is unneeded, since QEMU generates its
>>    own TPM2 table, which describes the TPM device's access method --
>>    TIS+Cancel (method 6).
>> - The SSDT from the driver is again unneeded. It provides (via the _DSM
>>    method) an ACPI-level API that the OS can use, for talking to the TPM
>>    device. An implementation detail of this ACPI method is that it raises
>>    an SMI, for entering the firmware at an elevated privilege level (= in
>>    SMM). Then, the actual TPM hardware manipulation, or even the TPM
>>    *software emulation*, is performed by the firmware, in SMM.
>> This approach is totally ill-suited for the QEMU virtualization stack.
>> For starters, none of the firmware code exist -- as open source anyway
>> -- that would actually handle such ACPI->SMM requests. Second, I'm sure
>> we don't want to debug TPM software emulation running in SMM guest
>> firmware, rather than an actual QEMU device model.
>> Once we have a real device model, accessed via IO ports and/or MMIO
>> locations, perhaps in combination with request/response buffers
>> allocated in guest RAM, the SMI/SMM implementation detail falls away
>> completely. Our TPM emulation would attain its "privileged / protected"
>> status simply by existing in the hypervisor (QEMU).
> Regarding the SMI/SMM: I think it will be needed for the TPM Physical
> Presence interface where ACPI gets a code from the user that it sends to
> the firmware and the firmware acts upon next reboot. SMM stores this
> code in a UEFI variable (EDK2) to protect it from modules executed by
> UEFI. I was trying to use a memory area (PPI memory device) for storing
> this code but it would not give the same protection for UEFI compared to
> the variable. I suppose the reason is that UEFI can execute (untrusted)
> code that could manipulate this memory area and cause unwanted changes
> to the TPM upon reboot by for example writing a code for clearing the
> TPM. How 'safe' would the BIOS be or any path from the BIOS until the OS
> kernel takes over? Can untrusted code be executed by something like a
> BIOS module (vgabios.bin and the like) and mess with that memory area? A
> grub module?

Yes, this is a correct assessment in my view. SMM provides protection to
platform firmware modules not only against the OS, but also against 3rd
party firmware components, such as boot loaders, various UEFI
applications, UEFI device drivers (loaded from disk or PCI card option
ROM BARs), and such.

SMRAM and the writeable pflash chip are two pieces of emulated hardware
whose (write) access is restricted to code executing in SMM.

This does not necessarily imply that QEMU should generate SMI-triggering
AML methods for the guest. Instead, the sensitive writeable register
block of the TPM chip that you added could be restricted to code
executing in SMM, similarly to pflash:

  -global driver=cfi.pflash01,property=secure,value=on

... On the other hand, I do realize this would take custom SMM code,
which is even worse than reusing the SMM variable service code.

Sigh, I *utterly* hate this. I maintain from my earlier email that
generating ACPI for this in QEMU is ill-suited. Minimally, the TPM2
table will conflict between edk2 and QEMU. edk2 will be *both* missing
code that's going to be necessary for the QEMU/OVMF use case, *and* it
will contain code that either conflicts with or is not dynamic enough
for the QEMU/OVMF use case.

> One other complication is the memory area that EDK2 requires for
> exchanging of data ('that code' for example) between ACPI and SMM. It's
> hard coded to 0xFFFF 0000. However, with SeaBIOS I cannot use this
> memory and there's this comment here: 'src/fw/shadow.c:// On the
> emulators, the bios at 0xf0000 is also at 0xffff0000'.
> So the point is SMM is needed for UEFI. QEMU would need to provide the
> ACPI code for it, which is basically a translation of the ACPI from EDK2
> so that this could work.


> To support SeaBIOS as well, we would have to be
> able to distinguish a BIOS from the UEFI on the QEMU level so that we
> could produce different ACPI

Yes and no,

> (no SMI and different OperationRegion than
> 0xFFFF 0000 for SeaBIOS),

"yes" with regard to the SMM difference, "no" with regard to the
operation region. We have an ACPI linker/loader command that makes the
firmware basically just allocate memory, and we have two other ACPI
linker/loader commands that (a) patch the allocation address into other
ACPI artifacts, (b) return the allocation address to QEMU (for device
emulation purposes), if necessary.

> *if* on a system with a BIOS the memory area
> can be considered to be safe (like that EDK2 variable). Otherwise I am
> afraid it's better to not support it in SeaBIOS and provide all
> necessary early TPM 2 operations via user interaction with the menu only.

FWIW, to my knowledge, the RH boot loader team is only interested in TPM
for UEFI.

Does Windows utilize TPM PPI when booted on a traditional BIOS computer?

> Comments ?

I think your analysis is correct, about SMM. Previously I missed that
specifically the Physical Presence operations needed protection.

My operating knowledge about the TPM had been that

  Components measure stuff into PCRs, and if any untrusted agent messes
  with those measurements, for example by directly writing to the PCRs,
  then the TPM will simply not unseal its secrets, hence such tampering
  is self-defeating for those agents.

While this might be correct (I hope it is correct!), the *PPI* part of
TPM appears entirely different. In fact I don't have the slightest idea
*why* PPI is lumped together with the TPM.

Can you explain in more detail what the PPI operations are, and why they
need protection, from what agents exactly? What is the purported
lifecycle of such PPI operations?


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