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Re: [Qemu-devel] investigating TPM for OVMF-on-QEMU
Re: [Qemu-devel] investigating TPM for OVMF-on-QEMU
Mon, 17 Jul 2017 02:03:25 +0000
This is a good summary.
One minor comment is:
1) Tcg2Pei/Dxe are arechitecture driver. We do not expect a platform modify
2) Tcg2ConfigPei/Dxe are platform sample driver. A platform may have its own
version based upon platform requirement. For example, if a platform supports
fTPM, it may use another Tcg2Config driver.
Back to enabling, I think the first important question is how to access TPM
If the TPM can expose FIFO interface or CRB interface defined in "Platform TPM
Profile Specification definition for TPM2.0", most current TPM2 drivers can be
The second question is about PEI variable.
I do not think PEI Variable is *required* component. Tcg2Pei has
gEfiPeiReadOnlyVariable2PpiGuid dependency, and I think we can remove it.
Tcg2ConfigPei has gEfiPeiReadOnlyVariable2PpiGuid dependency, but that is to
support TPM selection. The Tcg2Config driver is an optional driver, and it can
be customized by platform by design.
> -----Original Message-----
> From: Laszlo Ersek [mailto:address@hidden
> Sent: Saturday, July 15, 2017 2:04 AM
> To: Yao, Jiewen <address@hidden>; Stefan Berger
> <address@hidden>; Javier Martinez Canillas <address@hidden>;
> Peter Jones <address@hidden>; Marc-André Lureau
> <address@hidden>; Valluri, Amarnath
> Cc: edk2-devel-01 <address@hidden>; qemu devel list
> Subject: investigating TPM for OVMF-on-QEMU
> I spent most of today reading TPM related material (specs, guides and
> source code), and below I'd like to describe what I think should be our
> initial attack on enabling TPM on QEMU/OVMF.
> (1) Versions of UEFI abstractions regarding TPM (very roughly speaking):
> There have been three sets of specs for this, and accordingly,
> edk2's SecurityPkg/Tcg/ directory supports each set. We can call
> these TPM1, TrEE, and TPM2.
> - TPM1 was the initial TPM thing, supporting only SHA-1, and is now
> - TrEE was Microsoft's own update to the TPM1 software interfaces,
> supporting TPM2 hardware with more hash algorithms, and is now
> obsolete. Because,
> - TPM2 is basically the standardized version of TrEE, the most
> recent set of specs, and what we should focus on.
> (2) Drivers (and features) in edk2/SecurityPkg/Tcg.
> There are 19 modules under SecurityPkg/Tcg/. Let me categorize them.
> In each category, I'll try to list modules in loosely increasing
> dependency (or PI/UEFI phase) order.
> (2a) Modules that are obsolete because they are tied to TPM1:
> The TPM enablement instructions at
> are written up in terms of these modules, but that doesn't obsolete
> the article too much.
> (2b) Modules that are obsolete due to being tied to TrEE:
> There is a great Intel whitepaper called
> A Tour Beyond BIOS with the UEFI TPM2 Support in EDKII
> which is written in terms of TrEE and these modules, explaining how
> they supersede TPM1 and the modules under (2a).
> This whitepaper seems to apply to the most recent TPM2 stack as
> well (mostly just replace TrEE references with TPM2 references), so
> it is very useful.
> (2c) Modules that call themselves obsolete or deprecated (without being
> obviously tied to TPM1 or TrEE):
> (2d) Modules that we won't need due to no hardware support in QEMU:
> TCG's "Opal" seems to be about self-encrypting drives, see
> so these modules are not relevant for us.
> (2e) Modules that we should use. Again, in increasing order of
> dependence. And here I'll comment as well on what these do:
> Tcg2Config/Tcg2ConfigPei.inf -- Informs the firmware globally
> about the TPM device type. This
> module can perform device
> detection or read a cached value
> from a non-volatile UEFI
> Tcg2Pei/Tcg2Pei.inf -- Initializes the TPM device and
> measures the firmware volumes in
> the PEI phase into the TPM's
> platform config registers.
> Tcg2Dxe/Tcg2Dxe.inf -- Measures DXE phase (and later)
> modules into the TPM's PCRs, and
> also lets the OS boot loader
> measure things, by exposing the
> Tcg2Config/Tcg2ConfigDxe.inf -- Provides a Setup TUI interface to
> configure the TPM. IIUC, it can
> also save the configured TPM type
> for subsequent boots (see
> Tcg2ConfigPei.inf above).
> This driver stack supports the TIS (MMIO) hardware interface, which
> is advertized to the OS in the TPM2 ACPI Table's "start method"
> field with value 6. (The according macro is TPM2_START_METHOD_MMIO
> in the QEMU source code, and
> in the edk2 source code.)
> Including these drivers should result in a functional
> EFI_TCG2_PROTOCOL, which is what OS boot loaders primarily care
> about, as I understand.
> Importantly, the driver stack above requires PEI-phase variable
> access, therefore
> <https://bugzilla.tianocore.org/show_bug.cgi?id=386> must be solved
> (I have had patches for said BZ ready for a while. I've failed to
> upstream them thus far because a pflash-based varstore is a hard
> requirement for them. I think that's a natural requirement, but
> thus far my arguments haven't proved compelling enough.)
> Here I should mention some ACPI and hardware aspects. Under TPM1
> (whose ACPI table was called "TCPA"), the TPM events (measurements
> I think) were logged in a reserved memory area described by the
> TCPA table. Under TPM2, the "TPM2" ACPI table does no such thing,
> it only helps identify the communication characteristics of the
> device, and the event log itself is accessible to the OS boot
> loader via the EFI_TCG2_PROTOCOL.
> (If you are curious how a legacy BIOS boot loader is supposed to
> read the event log from a TPM2-only device (no "TCPA" table): I
> don't have the slightest clue.)
> I'm not sure about the exact characteristics of the virtual TPM
> that Stefan's swtpm project:
> combined with Amarnath's pending QEMU patches:
> will expose to the guest. What I do know is that the current QEMU
> solution, which mostly forwards a physical (host) TPM to the guest,
> produces a "TPM2" ACPI table if said host TPM device is TPM2. The
> "TPM2" table is exposed to the guest OS with OVMF's help, and has
> the following fields:
> - address of control area: zero
> - start method: 6 (TIS plus Cancel)
> - platform specific params: none.
> This implies that neither ACPI activation (method 2) nor Command
> Response Buffer activation (method 7) nor a combination of these
> two (method 8) is available in QEMU.
> And that should be just fine, because both Linux and the above
> Tcg2* modules appear to support this (from reading, not from
> (2f) Modules that we *could* use, but *should not*, at this point:
> MOR is "Memory Overwrite Request". It is a feature specified
> separately, in another TCG specification ("Platform Reset Attack
> Mitigation"), and it is optional for a firmware platform to
> support. (For example, as far as I can see, Linux doesn't even try
> to detect or use it.) If you care about the threat model and how
> MOR mitigates that threat, please read the spec on the TCG website.
> For initial TPM enablement in OVMF, we should avoid MOR support.
> The module above initializes the "MemoryOverwriteRequestControl"
> variable, which is one third of the MOR implementation.
> This is a complex driver that we *should not* use at this point:
> (2f1) It installs two ACPI tables. One of those is a "TPM2" ACPI
> table, according to the detected / configured TPM device (see
> under (2e). This "TPM2" table would conflict with -- or more
> precisely, duplicate -- the "TPM2" table already generated by
> QEMU, and installed by OVMF.
> (2f2) The other ACPI table is an SSDT which provides a _DSM (Device
> Specific Method) to the guest OS. Whenever the _DSM is
> called, the firmware is entered (via SMI/SMM), and the OS
> request is handled. Three sets of functions are provided:
> (2f2a) TCG Hardware Information. Not needed by the guest
> OS(es), the TIS+Cancel start method is sufficient.
> (2f2b) TCG Memory Clear Interface. This is the second third
> of the MOR feature I mentioned above. It saves the OS
> request in the "MemoryOverwriteRequestControl"
> variable (only writeable in SMM).
> Upon next boot, OVMF's PlatformPei would have to clear
> all memory -- this would be the last third of the MOR
> feature; see under:
> However, clearing all memory in OVMF's PlatformPei
> would be a *real* pain, as we are limited to the
> 32-bit address space there (yes, even if OVMF's PEI
> phase is built in 64-bit mode). In physical firmware,
> where DRAM takes actual hardware initialization, the
> clearing can likely occur without entering long mode
> and setting up large page tables (e.g., the memory
> controller itself could be programmed with IO
> accesses), so in this case physical firmware has it
> Again, Linux doesn't care about MOR.
> (2f2c) TCG Physical Presence Interface. The OS can queue TPM
> operations (?) that require Physical Presence, and at
> next boot, OVMF's PlatformBootManagerLib (in the BDS
> phase) would have to dispatch those pending
> operations. (The queueing again happens in protected
> non-volatile UEFI variables -- writeable only in SMM.)
> This is of dubious usefulness, and presents extra
> complication for OVMF. Said dispatching is supposed to
> occur *before* signaling the end-of-DXE event group
> (according to the Intel whitepaper mentioned above),
> but *after* the console is ready (according to the
> wiki article linked above). However, OVMF *already*
> signals the end-of-DXE event group in
> PlatformBootManagerBeforeConsole(), that is, *before*
> the console is ready. Many things are orchestrated
> around end-of-DXE, so moving around its signaling
> looks difficult.
> Also, while Linux appears to be capable of using
> Physical Presence, it is again optional, and I suggest
> to ignore it at first.
> In brief, by not including these two modules, we avoid a "TPM2"
> ACPI table duplication. We also turn off the Memory Overwrite
> Request and Physical Presence Interface features -- which are both
> optional, as far as I can see, and very messy for OVMF's platform
> (3) Drivers (and features) that are *not in edk2/SecurityPkg/Tcg:
> The Intel whitepaper discusses (and Peter also mentioned earlier)
> "dTPM" versus "fTPM".
> "dTPM" is basically TPM provided in publicly specified hardware,
> where the firmware can offer support, such as EFI_TCG2_PROTOCOL, but
> the OS can also directly drive the hardware. This is what QEMU
> offers with the TIS+Cancel start method (value 6). (The "Command
> Response Buffer" start method (value 7) would also qualify as
> "dTPM"). When the platform provides "dTPM", the _DSM method
> described above *may* be offered, but it is not required.
> "fTPM" is where the hardware is completely hidden from the OS, and
> is implemented fully in firmware. The corresponding start method
> values are 2 ("ACPI") and 8 ("ACPI with CRB"). In this case, the
> _DSM method is *required*.
> To my understanding, edk2 contains no "fTPM" implementation. The
> in-tree drivers recognize hardware that describes itself as
> TIS+Cancel (6) or CRB (7). Pure ACPI variants are neither recognized
> nor offered.
> I think TIS+Cancel / dTPM is the best match: the emulated TPM has to
> be implemented in virtual hardware (not just faked within the guest,
> in RAM), so that QEMU can secure the sensitive stuff from guest
> kernel level access.
> I'm going to link this post from
> <https://bugzilla.tianocore.org/show_bug.cgi?id=594>; please consider
> registering in the TianoCore BZ and subscribing to that bug. Discussion
> should occur on the list(s), but it's nice to capture separate threads
> and distilled ideas in the BZ.