Just for the funs I recently revived some older work with my PCEngines alix3d2 where I built an OE meta layer with a simple machine and kernel config to build images: meta-alix.
TPMs for all the boards!
I’ve got a soft spot for the older PCEngines WRAP board since it was the first platform I experimented on while building a home router / access point years ago. So meta-alix was fun work but nothing too crazy. While playing around with this I noticed that the alix3d2 has an exposed 20 pin header labeled ‘LPC’. Now that is interesting because I’ve got a few Asus branded Infineon TPMs laying about and they’ve got LPC connectors on them. A home wireless router with a TPM on it? Now that could be interesting.
Attaching an TPM designed to attach to a board on a 20 pin LPC connector should be pretty easy right? That’s what I thought too. But here we are 2 weeks later and I’m just now getting to write this up and I can’t say this work was 100% successful. But before I go too deep into the trials and tribulations let’s start with a bill of materials.
Bill of materials
To start out you’ll need a TPM and one designed to attach to your system on the LPC bus. TPMs are a PITA to buy really. There are 3 main companies that manufacture them but you can’t buy them direct. Thankfully there are some motherboard manufacturers out there that support the TPM via a “daughter-card” and from my experience this is mostly the high end manufacturers like Asus and Supermicro. I had 2 Asus TPMs laying around so this seemed like a good opportunity to put them to use. On Amazon these TPMs go for about $15 but when I bought mine almost a year ago they were less than half that.
The system that started out trying to attach this thing to is an alix3d2. I also picked up one of the newer PCEngines APU but *spoiler alert* only after I had serious problems getting the alix to work.
You’ll also need a soldering iron and the usual soldering / prototyping gear on hand (lights, wire, solder, magnifying glass etc). That’s right I said soldering. It’s been a while for me too. Like 10 years. Don’t worry there isn’t much too this and it was really fun.
As you’ve likely guessed by now, just because a system has an LPC connector doesn’t mean this thing is plug and play. The Asus TPM daughter card has pin 4 blocked / keyed and the ALIX doesn’t so that’s our first hint. The real data is in the respective pin diagrams. Finding these isn’t as easy as I’d hoped so I had to do some digging.
The docs for the ALIX systems are all on the PCEngines website so that part’s easy. The Asus TPM doesn’t seem to have any docs though. If you take the time to dig into the boards that support them though you’ll find the manuals for these boards have the pin assignment documented. I pulled down the manual for the P9D-WS and used this as a reference. Page 2-29 has what we’re looking for.
With the pin layouts in hand we can see clearly that plugging the TPM daughter card directly into the board isn’t gonna happen. I’ll reproduce the layouts here so we can view them side by side:
|Asus TPM||PCEngines LPC|
There’s basically no overlap in the pin layouts here except for a few ground connections. This blew my mind at first but after searching through the Intel Low Pin Count Interface Specification it turns out that this bus was intended for use on-board only and so there’s no pin layout specified for external connectors. First mystery solved. Now let’s figure out how we’re gonna wire this thing up.
To the breadboard!
This isn’t going to be as easy as “plug and play” but it’s not far off. We just need to connect the right pins. With the pin map above and a little help from the spec (to get the minimum required connections) we can pull out our breadboard and prototype this thing.
If you’re like me you’ll have to go out and buy materials as you need them. Luckily I live minutes away from HSC Electronic Supply which is an amazing surplus electronic shop. After an hour or 3 poking around the piles of old electronic gear I managed to scrounge up a 20 pin ribbon cable with a connector that looked like it might fit on my breadboard. With a 20 pin DIP ribbon cable connector I had what I needed to connect the alix to the breadboard.
Next was to get the TPM daughter card wired up to the breadboard. This was harder than I expected. I couldn’t easily find a connector that would suit this purpose that didn’t require waiting for shipping. So I soldered some wires up to breakaway headers and rigged up a horrible TPM-to-breadboard connector. Then we just hook up the two using the following mapping:
|1||1||PCICLK / LCLK: 33MHz clock|
|3||11||LFRAME#: Transaction signal|
|5||13||LRESET#: Bus reset. AKA PCIRST#|
|7||9||LAD3: Data lane|
|8||7||LAD2: Data lane|
|9 & 15||18||3 Volts DC|
|10||5||LAD1: Data lane|
|11||3||LAD0: Data lane</td|
|16||19||SERIRQ: Serialized interrupt signal|
After some fiddling (kicking, screaming and burning myself with a soldering iron) this is what it looked like:
Now it SHOULD have worked. These are the right connections. But on the alix3d2 I got no love. I didn’t actually get this set-up to work till my apu1d showed up in the mail 3 days later. For whatever reason the external LPC on the alix3d2 just doesn’t work as advertised. Without an oscilloscope I can’t debug much beyond whether the voltage and ground pins are OK (and they are) so for now that will remain a mystery. So the alix3d2 is out and the apu1d is in.
Anyways we can do better than this bootleg breadboard setup. Let’s see about cleaning it up.
Clean it up
The wiring above was super flaky and that shouldn’t be a surprise. I didn’t get the length of each wire exact and the pins slipped around a bit in the plastic. I ordered some IDC breakout helpers from Adafruit but they were garbage. They plug into the breadboard fine but the pins aren’t long enough and they just pop back out immediately.
So again I hacked up another connector out of DIP/DIL headers and some breakaway headers spaced to span the gap in the breadboard. This is generally a bad idea since the solder is what’s holding the whole thing together but it worked out pretty OK:
Packaging for the APU enclosure
After convincing ourselves that the wiring above is right on the breadboard we need to clean this up so that it fits in the enclosure with the APU. There’s not a lot of space in the PCEngines recommended case1d2 but there’s enough if we’re sufficiently inventive. And by “inventive” I mean “you’ve got a hacksaw”.
Start out by removing the female header from the TPM and trim back the connector pins. If we flip this header on it’s side we can use it to mount the TPM once we reconnect it. This would require either unblocking pin 4 on the connector or cutting pin 4 off of the APU board. Since pin 4 on the APU is ground anyways this shouldn’t be a problem.
I used a 20 pin DIP to ribbon cable connector for my setup. I sanded down the daughter board to expose the copper on the base which happens to be ground and connected this with the even pins on the connector up through 12. This proved to be a pretty solid base as it holds the daughter board nice and tight to the connector.
Then we just cut wires and solder pins per the table above. The wire I had on hand was 28 gauge which was a bit too big and the soldering job is straight up ugly in spots but it’s the first bit of soldering I’ve done in 10 years so that’s good enough for me. I’ve got another TPM on hand so I’ll have another go now that I’ve had some practice.
I used both a Debian install with the tpm-tools package to test this as well as the core-image-tpm from meta-measured. I’d recommend sticking with Debian unless you feel like falling down the rabbit hole of an OE build. The important thing to keep in mind is that the APU BIOS doesn’t support the TPM so it won’t do the necessary setup for us.
The BIOS is supposed to do a number of things to set things up so that the OS can use the TPM. This includes running the TPM self test, enabling it and setting up ACPI entries to make it easy for the OS to talk to it. With the stock BIOS on the APU we won’t get any of this. Thankfully the number of platforms that have implemented TPM support wrong in the BIOS over the years is quite high so the Linux TPM TIS driver can do all of this for us if we give it the right parameters:
root@apu:~# modprobe tpm_tis force=1 [ 74.027383] tpm_tis tpm_tis: 1.2 TPM (device-id 0xB, rev-id 16) [ 74.063388] tpm_tis tpm_tis: Issuing TPM_STARTUP [ 74.260392] tpm_tis tpm_tis: TPM is disabled/deactivated (0x7) [ 74.308465] genirq: Flags mismatch irq 4. 00000080 (tpm0) vs. 00000000 (serial) [ 74.315956] tpm_tis tpm_tis: Unable to request irq: 4 for probe [ 74.436459] genirq: Flags mismatch irq 8. 00000080 (tpm0) vs. 00000000 (rtc0) [ 74.443753] tpm_tis tpm_tis: Unable to request irq: 8 for probe
modinfo command will tell you all of the gory details about what these parameters do if you’re interested. The short version is that
force=1 causes the driver to ignore ACPI and probe for the TPM device. You can also add the
interrupts=0 argument to disables interrupts which will get rid of all of the
genirq errors. After this you should see
/dev/tpm0 appear magically. You can then start
tcsd and get some version info out of the TPM:
root@apu:~# tpm_version TPM 1.2 Version Info: Chip Version: 126.96.36.199 Spec Level: 2 Errata Revision: 2 TPM Vendor ID: IFX Vendor Specific data: 0313000b 00 TPM Version: 01010000 Manufacturer Info: 49465800
You won’t be able to do much more than this though since the BIOS hasn’t enabled the TPM for us. We’ll get past this in my next post.
One thought on “Attaching a TPM on the LPC”
Nice article, thanks. The problem is exactly the LPC bus. As it is not encrypted the compete set of information can be captured on it and later decoded using a simple program. So the potential attacker with physical access to the device can obtain keys, PCR updates, whatever. See my post http://fis-cz.blogspot.cz/2017/05/tpm-12-is-not-secrue-enough-without-pin.html