My work to calculate PCR[18] from the last post was missing one big piece. I took a short cut and parsed the MLE hash out of the SINIT to MLE data table. This was a stop gap.
The MLE wasn’t being measured directly. We were still extracting the measurement as taken by the SINIT which is a binary blob from Intel. We don’t have a choice in trusting this blob from Intel but we can verify the measurements it takes. With this in mind I’ve gone back and added a tool to the pcr-calc module to calculate the MLE hash directly from the MLE.

The MLE Hash

Calculating the MLE hash is a bit more complicated than just hashing the ELF binary that contains it. There’s already a utility that does this in the tboot project though it’s pretty limited as it only dumps out the hash in a hex string. My end goal is to integrate this work into a bitbake class so having a python class to emit a hash object containing the measurement of the MLE is a lot more convenient.

In the pcr-calc project I’ve added a few things to make this happen. First is a class called mleHeader that parses the MLE header. This is just more of the mundane data parsing that I’ve been doing since this whole thing started. Finding the MLE header is just a matter of searching for the magic MLE UUID: 5aac8290-6f47-a774-0f5c-55a2cb51b642. Having the header isn’t enough though. The MLE must be extracted from the ELF and this is particularly hard because I know nothing about the structure of ELF files.

To do the extracting I basically ported the mlehash utility from tboot to python. The MLE is actually stored in the ELF file program header. This requires parsing and extracting the PT_LOAD segments. Writing a generic ELF parser is way beyond the scope of what I’m qualified to do but thankfully Eli Bendersky already has a handle on this. Check out pyelftools on his github page. You can download the package for pyelftools through the python package system like so:

$ pip install pyelftools

I’ve not yet integrated a check for this package into the pcr-calc autotools stuff yet but I’ll get around to it.

So in pcr-calc, the MLEUtil class does a few things. First it unzips the ELF file if necessary. Second, the ELFFile class from pyelftools is used to extract the PT_LOAD segments from the ELF. These are copied to a temporary file and the excess space is zero-filled. Once the ELF is extracted we locate the MLE header by searching for the UUID above. This header is represented and parsed by the mleHeader object.

The end goal is to calculate the SHA1 hash of the MLE. The fields in the header we need to do this are mle_start_off and mle_end_off. These are the offset to the start and end of the MLE respectively. Both offsets are relative to the beginning of the extracted ELF. The hash is then simply calculated over the data in this range.

Housekeeping

With the objects necessary to calculate the MLE hash done I went back and updated the pcr18 utility. Now instead of parsing the hash out of the TXT heap it now hashes the MLE directly. The mlehash program is constructed in a similar way but it is limited to calculating the MLE hash only.

Conclusion

A significant amount of the work in calculating the MLE hash was just code reading, firstly to understand how to extract and measure the MLE, second to understand how use the pyelftools package. Using pyelftools means that pcr-calc has a new dependency but it’s a lot better than implementing it myself. Working with pyelftools has been beneficial not only in that it saves me effort but it’s also an excellent example to work from. pcr-calc is my first attempt at implementing anything in python and it shows. Having poked around in pyelftools a little bit I’ve realized that even though my code “works” it’s pretty horrible. Future efforts to “clean up” pcr-calc will model significant portions of it after the code in pyelftools.

Having completed calculating the MLE hash we’ve taken a big step forward in our effort to construct future PCR values by measuring the individual components. It’s the last step in removing dependence on the extracted heap. We can now calculate PCR[18] and PCR[19] without any knowledge of or access to the deployed platform hardware and that’s pretty great. PCR[17] by contrast contains a whole bunch of stuff like the STM hash that’s independent from the Linux OS being run. For now I’m happy to assume PCR[17] is static for a system and doesn’t need to be calculated in the build system.

Eventually I’d like to extend pcr-calc to include mechanisms for ingesting an LCP and calculating PCR[17] but that’s a long way off. Instead, my next steps will be to clean up the pcr-calc code and integrating it into the meta-measured OE layer. The end goal here is to produce a manifest that a 3rd party (an installer or a remote system) can use to either seal secrets to a future platform state or for appraising an attestation exchange. More on this front next.

In my first post on the subject, I indicated calculating PCR[18] and PCR[19] was significantly easier than PCR[17]. If you read my last post on calculating PCR[17] you’ll see why. Since then I’ve gone through and sorted out calculating the remaining two PCRs and it is in fact pretty easy (few!).

PCR[18]

Unlike PCR[17], 18 and 19 are mostly just hashes of boot modules. Unfortunately the MLE Developers Guide again is a bit vague here and maybe a bit misleading even. I’m starting to think that the guide on the Intel website is out of date.

The state of PCR[18] is defined in section 1.9.2. From the text:

PCR 18 will be extended with the SHA-1 hash of the MLE, as reported in the
SinitMleData.MleHash field.

This is true but it’s only the first extend operation. There can be a second and by default (default tboot behavior) there is. The second hash is of the first boot module after the MLE. The is the module tboot executes after the SINIT.

So what’s the MLE (Measured Launch Environment) anyways? The MLE is the thing that does the measured launch. Pretty much it’s the code that executes GETSEC[SENTER] and hands control over to the SINIT ACM, that’s tboot. After a successful measured launch this hash will be in the TXT heap in the SINIT to MLE data table. The field is called the MleHash.

Like I said in my first post: this isn’t about blindly trusting the hashes in the heap though. We want to be able to isolate the thing being measured so if the MLE is tboot then we should be able to take the SHA1 hash of the tboot binary and get the same value … or not. There’s actually a structure within tboot that defines the MLE. The hash of this structure is what’s in the MleHash filed of the SINIT to MLE data table.

There’s already a stand-alone program in the tboot code to calculate this hash for use in the LCP. I’ve not yet looked too deep into how this is calculated though. For now, if you’re interested, you can use this tool to calculate the MleHash and compare this to the data reported in your TXT heap or the txt-stat output. Mine looks like this:

  $ ./lcp_mlehash -c "logging=serial,vga,memory" ~/txt-test/modules/tboot.gz
  5b d5 12 72 1e 07 5e 31 4d 8d e5 2e 5f b9 10 04 d4 00 e7 27

A very important thing to note here is that we pass not only the module to this program but the arguments passed to it as well.

So the MLE Dev Guide indicates that PCR[18] will be extended with the MleHash we’ve just now calculated. But if you check the status of your PCR[18] after a measured launch you’ll realize that it can’t be calculated with just the MleHash. So the MLE Dev Guide isn’t really the best documentation for this stuff I guess. You’re much better off reading the README in the tboot sources.

In the README it’s clearly stated that PCR[18] is extended not only with the MleHash but also with the hash of the first boot module. The bit that’s missing of course is a description of how modules are hashed. The exact algorithm for doing this isn’t in the Dev Guide or the README though, it’s in the tboot source code in the file tboot/common/policy.c.

A module hash is simply the hash of the command line passed two the module concatinated with the module itself (remember the two parameters passed to the lcp_mlehash above). There isn’t much to processing the command line but if you’re interested you can check out the code. I’ve added a utility called module-hash to automate this to the pcr-calc library. Using the notation we’ve been using up till now this could be represented as:

  module_hash = SHA1 (cmdline | module)

You can invoke the utility to perform this calculation like so:

  module-hash --cmdline tboot.cmdline --module tboot.gz

Combine this with the MleHash and the full PCR[18] calculation is as follows:

  PCR[18]_1 = sha1 (PCR[18]_0 | MleHash)
  PCR[18]_2 = sha1 (PCR[18]_1 | sha1 (cmdline | module))

there are two extends required to calculate PCR[18]. And as a reminder: PCR[18]_0 is the PCR before any extend operations so it contains 20 bytes of 0’s. In my example setup the module that’s hashed and extended into PCR[18] after the MLE is just the linux kernel file vmlinuz.

The pcr-calc project has a pcr18 utility too that wraps the MleHash and the hashing of the module for convenience. It gets the MleHash from the heap still but the next logical step is to calculate it directly from the tboot binary and the commandline. Invoke pcr18 like so:

  $ pcr18 --module vmlinuz --cmdline vmlinuz.cmd txtheap.bin

The --cmdline argument is a file containing a single line of text which is parsed as the parameters to the module.

PCR[19]

PCR[19] is just as trivial to calculate and again the Dev guide is little help. There’s nothing mentioned of this PCR in the Dev Guide but the LCP dictates what gets hashed and stored in it.
The default tboot policy extends PCR[19] with the hash of all remaining modules. That’s all of the boot modules that aren’t an SINIT ACM, the MLE or the first module.

This leaves the initrd on my system (yours may be different and may have more than one module). If the module is compressed it must be decompressed before it’s hashed. The calculation of PCR[19] can be described as follows:

  PCR[19]_1 = sha1 (PCR[19]_0 | sha1 (cmdline_1 | module_1))
  PCR[19]_n = sha1 (PCR[19]_n-1 | sha1 (cmdline_n | module_n))

Remember that module_0 is tboot so it’s measured as part of PCR[18] so we start here with cmdline_1 and module_1.

The pcr-calc project has yet another program pcr19 that automates the calculation of PCR[19]. The calling convention is a bit awkward here and I’ll probably have to come up with something better:

  $ pcr19 module1.cmd,module1 module2.cmd,module2

The arguments are all positional. They’re ordered pairs of files with the first being a file containing the command line text and the second being the module itself. This means your file paths can’t have commas in them.

Conclusion

When I started out on this quest to pre-calculate the DRTM PCRs as populated by the Intel TXT hardware and tboot I knew this would be painful but I didn’t realize how much. It really was an exercise in masochism. Now that I’ve jumped through the hoops and done the digging necessary to understand the process, I’ve got a pretty good understanding of what’s required to move to the next phase of this project.

My prevous work with OE and the meta-measured layer is the groundwork. It produces a system image that will do a TXT measured launch but the PCR values after boot are still a mystery. I had hoped to be able to calculate all of these measurements in the build, but access to the TXT heap from the target device isn’t realistic. From the work discussed above however, it looks like PCR[18] and PCR[19] can be calculated reliably.

The remaining work is to calculate the MleHash, though the existing tool in tboot is extremely close to what we need. Combined with the tools from pcr-calc this is likely sufficient. All of this will need to be combined and integrated into the meta-measured layer likely as part of an image class. Sounds like my next task is to clean this stuff up and revive meta-measured.

When I left off my last discussin of tboot PCR calculations I gave a quick intro but little more. In this post I’ll go into details for calculating the first of them: PCR[17].

There have been a number of discussions with regard to calculating or verifying PCR values on the tboot-devel mailing list and they were extremely useful in writing this code and post. These all fell a bit short of what I wanted to accomplish in that all approaches extracted hashes from the output of the txt-stat program (the tboot log) and used those hashes to re-construct the PCR values. I wanted to construct all hashes manually, to measure and account for the actual things that TXT and tboot were measuring and storing into the PCRs and to do this independent of an actual measured launch. Basically this translates to isolating the things being measured, extracting them (if possible) and use them to reconstruct the PCR value on any system, like a build server or an external verifying party.

The process is pretty straight forward, though time consuming, and the specification is phrased in such a way as to force some guess and check. There’s even a bit of a trick in the end which requires that we go digging around in the tboot source code which is always fun. I’ll also present a bit of code that will automate the calculation for you so if you’re anxious and don’t want to read any more you can go straight to the code which can be found here: .

DISCLAIMER: The code in the pcr-cal git repo is very much a work-in-progress and should be considered unstable at best so YMMV.

The spec that defines the DRTM specific PCRs is the “PC Client Implementation for BIOS”. These are PCRs 17 through 20. Their individual use however is hardware specific and on Intel hardware, the definitive source of data on what gets extended into which of these PCRs as part of establishing a DRTM is a document titled “Intel® Trusted Execution Technology (Intel® TXT) Software Development Guide: Measured Launched Environment Developer’s Guide”.

Quite a mouth full. Anyways section 1.9.1 covers PCR[17] but the details of what various bits are measured are spread out over the document. A default tboot configuration will cause 3 extends to this PCR so we’ll break this post up into 3 sections, each one describing the hashes that go into the 3 consecutive extend operations.

First extend: SINIT ACM

The first thing that’s extended into PCR[17] is the hash of the SINIT ACM. This is a binary blob that Intel ships which is used by tboot to establish the DRTM on a platform. The binary code in the ACM is chipset specific so there are a number of ACMs out there to chose from. tboot automates the process so if you’re unsure which ACM is the right one for your platform you can configure your bootloader to load every ACM and tboot will pick the right one. This will slow your boot process down considerably though and selecting the proper one isn’t hard with a bit of reading so don’t be lazy.

With the right ACM in hand you’d think it would be a simple matter of calculating the sha1 hash of the file and extending that into PCR[17]. That’s not the case though. There are two little details that must be sorted first.

Depending on the version of the ACM you’re using the hash algorithm may be sha1 or it may be sha256. ACMs version 7 or later will use sha256, while earlier versions will use sha1. The current version of the ACM format is 8 so most modern hardware will need a sha256 hash (not to mention that most OEM implementations of TXT 3 years or older never worked in the first place … snap!).

Further, there are some fileds in the ACM that aren’t included in the hash. The logic behind this escapes me but the apendix A.1.2 specifies that some fields are omitted. Quoting the spec: “Those parts of the module header not included are: the RSA Signature, the public key, and the scratch field.” That sounds like 3 fields from the ACM right? Wrong: there’s a 4th field omitted as well and that’s the RSA exponent. I guess they meant for the exponent to be included in the definition of “public key”? Thanks for being explicit.

Anyways omit the fields: RSAPubKey, RSAPubExp, RSASig and the Scratch space, got it. To omit these fields from the hash we’ve gotta parse the ACM. I’ll cover this code at the end.

Finally the 32 bits that make up the EDX register which hold the flags passed to the GETSEC[SENTER] instruction are appended to the hash of the ACM. We represent PCR[17] at the first extend operation thusly:

PCR[17]_1 = Extend(PCR[17]_0 | SHA256 (ACM | EDX))

where PCR[17]_0 is the state of PCR[17] at time = 0. PCRs are initialized to 20bytes of 0’s so PCR[17]_0 is 20 bytes of 0’s.

Second Extend: Heap Data

The second extend to PCR[17] includes various bits of data from the TXT heap. Appendix C describes the TXT heap as a contiguous region of memory set asside by the BIOS for use by ‘system software’ (aka BIOS) to pass data to the SINIT ACM and the MLE. PCR[17] is extended with the sha1 hash of either 6 or 7 concatenated pieces of data depending on the version of the ACM. The following fields are concatenated together and their sha1 hash is extended into PCR[17] for the second extend:

1. BiosAcmId
2. MsegValid
3. StmHash
4. PolicyControl
5. LcpPolicyHash
6. OsSinitCaps or 4 bytes of 0’s as specified by the LCP (more on this next)

If the SINIT to MLE data table version is 8 or greater an additional 4 bytes are appended representing the processor S-CRTM status. These 4 bytes are in the ProcScrtmStatus field in the SINIT to MLE data table.

The second extend to PCR[17] could be represented as follows for SINIT to MLE data table versions < 8:

PCR[17]_2 = sha1 (PCR[17]_1 | sha1 ( SinitMleData.BiosAcm.ID | SinitMleData.MsegValid |
                                     SinitMleData.StmHash | SinitMleData.PolicyControl |
                                     SinitMleData.LcpPolicyHash | (OsSinitData.Capabilities, 0)))

For SINIT to MLE data table versions >= 8:

PCR[17]_2 = sha1 (PCR[17]_1 | sha1 ( SinitMleData.BiosAcm.ID | SinitMleData.MsegValid | 
                                     SinitMleData.StmHash | SinitMleData.PolicyControl |
                                     SinitMleData.LcpPolicyHash | (OsSinitData.Capabilities, 0) |
                                     SinitMleData.ProcessorSCRTMStatus))

where we use the notation (OSSinitData.Capabilities, 0) to represent a choce made between appending the value of OsSinitData.Capabilities or 4 bytes of 0’s depending on the state of the LCP policy control field.

The astute reader is likely wondering: “How do I get these values out of the TXT heap … and where do I even get the TXT heap from?” Both are very good questions. Getting at the TXT heap isn’t too difficult. You’re on a Linux system presumably with root access. The TXT heap is a region of memory like any other and you only need to know the offset where it resides and how to determine it’s size. Both the offset and the heap size are obtained from the TXT public registers which are mapped to well known memory addresses (read the spec if you’re really interested).

In the git repo linked above I’ve written a simple utility to parse and output the TXT Heap: txt-heapdump. You can run this utility to display the contents of the heap on the system you wish to calcuclate PCR[17] in a human readable form:

$ txt-heapdump --mmap --pretty

You can also use it to obtain the heap as a binary file:

$ txt-heapdump --mmap > txtheap.bin

You can then parse the binary file to display the heap in a human readable form and it’ll look just like it did coming straight from /dev/mem::

$ txt-heapdump --pretty -i txtheap.bin

Once you have the heap as a file you can use the pcr-calc library to parse and extract various bits. Again, I’ll present the utility that does it all for you at the end. But first, the third and final extend …

Third Extend: Launch Control Policy (LCP)

You’d expect that all of the values that are hashed as part of PCR[17] are discussed in the spec under section “PCR 17” … and you’d be wrong. A couple of sections deeper where the LCP is discussed, you’ll find a description of how the LCP policy is measured and it turns out that this measurement gets extended into PCR[17] as well! There are a number of rules laid out in this section for how the system behaves when there’s no ‘Supplier’ or ‘Owner’ LCP present. Specifically the spec states:

As a matter of integrity, the LCP_POLICY::PolicyControl field will always be extended into PCR 17. If an Owner policy exists, its PolicyControl field will be extended; otherwise the Supplier policy’s will be. If there are no policies, 32 bits of 0s will be extended.

I’ve not gone through this section with a very thorough eye so I’m not an authority here, but tboot seems to ignore these rules and instead loads a default policy when there isn’t one in the TPM NV RAM. Not saying this is good or bad, right or wrong, just pointing out that this is what tboot does and it was something that I had to figure out in order to calculate the value of PCR[17] independently on my test systems.

So my goal here is to calculate PCR values. If your system is like mine and both you (the ‘Owner’) and the ‘Supplier’ (your OEM) didn’t provide an LCP, how do we measure the default policy from tboot? The only thing I could come up with is to pull apart the tboot code and copy the hard-coded structures into a C program and then dump them to disk in binary. The hash of this file is the one we need to extend into PCR[17] along with the LCP PolicyControl value. I’ve added a class to the pcr-calc library to parse the necessary parts of the binary LCP to support this operation.

The program that dumps the binary LCP from tboot is: lcp_def I’ve kept this utility in the pcr-calc project to reproduce the LCP on demand. I considered only keeping around the LCP binary in a data file but in the event that the default tboot policy changes in the future I wanted to keep the program around to dump the binary structures. When executed this program just dumps the binary policy so you’ll have to redirect the output:

$ lcp_def > lcp.bin

Final PCR[17] Calculation

Now that we’ve figured out how to do all three independent extend operations and we’ve collected the heap and LCP blobs, we can calculate the final state of PCR[17]. I’ve automated this in the program: pcr17 (very creative name I know). Assuming your heap is in txtheap.bin and your LCP is in lcp.bin your SINIT ACM file is named sinit.acm you should invoke the program as follows:

$ pcr17 -i txtheap.bin -l lcp.bin sinit.acm

Your output should look something like this:

$ ./bin/pcr17 -i ../txt-data/txtheap.bin -l ../txt-data/lcp_def.bin ../3rd_gen_i5_i7_SINIT_51.BIN 
first extend: SINIT ACM hash
  extending with: 0fcc099f81549da4836d492afb8ab2e303cecfa1
  PCR[17] before extend: 0000000000000000000000000000000000000000
  PCR[17] after extend: 8d3dd5c8e795dfac5dbfa9859310b2bcea36d347
second extend: TXT heap data
  append BiosAcmId:
    8000 0000 2010 1022 0000 b001 ffff ffff 
    ffff ffff 
  append MsegValid_Bytes:
    0000 0000 0000 0000 
  append StmHash:
    0000 0000 0000 0000 0000 0000 0000 0000 
    0000 0000 
  append PolicyControl_Bytes:
    0000 0000 
  append LcpPolicyHash:
    0000 0000 0000 0000 0000 0000 0000 0000 
    0000 0000 
  append Capabilities_Bytes: False
    Hashing 4 bytes of 0s in place of OsSinit.Capabilities
  append ProcScrtmStatus_Bytes:
    0000 0000 
  extending with: 7e0cdad3b8d9c344ab89657efdbfa638d1b25978
  PCR[17] before extend: 8d3dd5c8e795dfac5dbfa9859310b2bcea36d347
  PCR[17]: bfa4421b49f6ab899157ba6ee8fec3c5c5abf4ab
third extend: LCP
  lcp hash: ab41624e7d71f068d48e1c2f43e616bf40671c39
  polctrl: 1
  extending with: 9704353630674bfe21b86b64a7b0f99c297cf902
  PCR[17] before extend: bfa4421b49f6ab899157ba6ee8fec3c5c5abf4ab
  PCR[17] after extend: 57a5f1b245ac52614498a728efe7f741b4dc3ebf

PCR[17] final: 57a5f1b245ac52614498a728efe7f741b4dc3ebf

Currently the program will dump the hashes and PCR states after each extend along with the actual sha1 that should be in PCR[17] after a successful TXT measured launch. Take a look at the code if you’re interested in the details. This was as much an exercise for me in learning a bit of python as it was about the actual end result. If given the choice again I’d have implemented this in C just because it’s much easier to deal with binary values and memory ranges in C than in Python. Then again this may just be that I know C better than I know Python so YMMV.

Conclusion

As you’ve probably noticed I was only partially successful in my goal. All of the data from the TXT heap that are extended in to PCR[17] are themselves hashes of things we can’t access. Most of these hashes are all 0’s though denoting that the BIOS implementer opted out of implementing that feature (you’ll have an STM on your system one of these days but don’t hold your breath). The only one that’s actually present is the BiosAcmId but I’d expect in the future for the other fields to be populated as well.

This is just another instance of binary blobs making their way into the TCB of our software systems. We’ve had to deal with these in various forms over the years: binary drivers, firmware and BIOS code. Intel and other chip manufacturers have been making their hardware extensible using firmware and microcode for a while now so it’s no surprise that these things have made their way into the TCB. The good news is that they’re being measured and even if we can’t get our hands on the code, or even the binary blob on account of it being embedded in some piece of hardware, we can still identify them by their hash. The implications for trust aren’t great but it’s a start.