Post Syndicated from Matthew Garrett original https://mjg59.dreamwidth.org/67602.html
TPMs contain a set of registers (“Platform Configuration Registers”, or PCRs) that are used to track what a system boots. Each time a new event is measured, a cryptographic hash representing that event is passed to the TPM. The TPM appends that hash to the existing value in the PCR, hashes that, and stores the final result in the PCR. This means that while the PCR’s value depends on the precise sequence and value of the hashes presented to it, the PCR value alone doesn’t tell you what those individual events were. Different PCRs are used to store different event types, but there are still more events than there are PCRs so we can’t avoid this problem by simply storing each event separately.
This is solved using the event log. The event log is simply a record of each event, stored in RAM. The algorithm the TPM uses to calculate the PCR values is known, so we can reproduce that by simply taking the events from the event log and replaying the series of events that were passed to the TPM. If the final calculated value is the same as the value in the PCR, we know that the event log is accurate, which means we now know the value of each individual event and can make an appropriate judgement regarding its security.
If any value in the event log is invalid, we’ll calculate a different PCR value and it won’t match. This isn’t terribly helpful – we know that at least one entry in the event log doesn’t match what was passed to the TPM, but we don’t know which entry. That means we can’t trust any of the events associated with that PCR. If you’re trying to make a security determination based on this, that’s going to be a problem.
PCR 7 is used to track information about the secure boot policy on the system. It contains measurements of whether or not secure boot is enabled, and which keys are trusted and untrusted on the system in question. This is extremely helpful if you want to verify that a system booted with secure boot enabled before allowing it to do something security or safety critical. Unfortunately, if the device gives you an event log that doesn’t replay correctly for PCR 7, you now have no idea what the security state of the system is.
We ran into that this week. Examination of the event log revealed an additional event other than the expected ones – a measurement accompanied by the string “Boot Guard Measured S-CRTM”. Boot Guard is an Intel feature where the CPU verifies the firmware is signed with a trusted key before executing it, and measures information about the firmware in the process. Previously I’d only encountered this as a measurement into PCR 0, which is the PCR used to track information about the firmware itself. But it turns out that at least some versions of Boot Guard also measure information about the Boot Guard policy into PCR 7. The argument for this is that this is effectively part of the secure boot policy – having a measurement of the Boot Guard state tells you whether Boot Guard was enabled, which tells you whether or not the CPU verified a signature on your firmware before running it (as I wrote before, I think Boot Guard has user-hostile default behaviour, and that enforcing this on consumer devices is a bad idea).
But there’s a problem here. The event log is created by the firmware, and the Boot Guard measurements occur before the firmware is executed. So how do we get a log that represents them? That one’s fairly simple – the firmware simply re-calculates the same measurements that Boot Guard did and creates a log entry after the fact[1]. All good.
Except. What if the firmware screws up the calculation and comes up with a different answer? The entry in the event log will now not match what was sent to the TPM, and replaying will fail. And without knowing what the actual value should be, there’s no way to fix this, which means there’s no way to verify the contents of PCR 7 and determine whether or not secure boot was enabled.
But there’s still a fundamental source of truth – the measurement that was sent to the TPM in the first place. Inspired by Henri Nurmi’s work on sniffing Bitlocker encryption keys, I asked a coworker if we could sniff the TPM traffic during boot. The TPM on the board in question uses SPI, a simple bus that can have multiple devices connected to it. In this case the system flash and the TPM are on the same SPI bus, which made things easier. The board had a flash header for external reprogramming of the firmware in the event of failure, and all SPI traffic was visible through that header. Attaching a logic analyser to this header made it simple to generate a record of that. The only problem was that the chip select line on the header was attached to the firmware flash chip, not the TPM. This was worked around by simply telling the analysis software that it should invert the sense of the chip select line, ignoring all traffic that was bound for the flash and paying attention to all other traffic. This worked in this case since the only other device on the bus was the TPM, but would cause problems in the event of multiple devices on the bus all communicating.
With the aid of this analyser plugin, I was able to dump all the TPM traffic and could then search for writes that included the “0182” sequence that corresponds to the command code for a measurement event. This gave me a couple of accesses to the locality 3 registers, which was a strong indication that they were coming from the CPU rather than from the firmware. One was for PCR 0, and one was for PCR 7. This corresponded to the two Boot Guard events that we expected from the event log. The hash in the PCR 0 measurement was the same as the hash in the event log, but the hash in the PCR 7 measurement differed from the hash in the event log. Replacing the event log value with the value actually sent to the TPM resulted in the event log now replaying correctly, supporting the hypothesis that the firmware was failing to correctly reconstruct the event.
What now? The simple thing to do is for us to simply hard code this fixup, but longer term we’d like to figure out how to reconstruct the event so we can calculate the expected value ourselves. Unfortunately there doesn’t seem to be any public documentation on this. Sigh.
[1] What stops firmware on a system with no Boot Guard faking those measurements? TPMs have a concept of “localities”, effectively different privilege levels. When Boot Guard performs its initial measurement into PCR 0, it does so at locality 3, a locality that’s only available to the CPU. This causes PCR 0 to be initialised to a different initial value, affecting the final PCR value. The firmware can’t access locality 3, so can’t perform an equivalent measurement, so can’t fake the value.
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