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I am working on the ATMEL TPM device on an embedded platform. While experimenting with the extend operation of the PCR, I extended the PCR 0 with a string of 20 bytes. As mentioned in the TPM specification, the PCRs 0-15 are resettable on reboot. On reboot, PCR 0 was not reset and the PCRs 1-15 also got modified. Here's the flow of operation which I performed and the corresponding output.


# cat /sys/class/misc/tpm0/device/pcrs

PCR-00: FD 89 A2 DE 1A 91 D7 A2 2B D1 78 7A A7 C2 77 9D E0 99 F7 C0
PCR-01: 49 20 44 4B 1E AF B2 AA 4A C1 2B D1 44 2B 82 1F 52 EC E7 4B
PCR-02: 38 53 A8 EF 61 83 59 ED 7F 7F 2E CC 7B C8 D2 F3 87 EB 7C 55
PCR-03: 2C 2F B4 2A 15 36 B2 28 C6 01 40 D8 64 D7 30 7F AA 6D 91 54
PCR-04: 2E CF 07 F9 C7 30 B4 4C EE 19 7B 0D 36 4E EE 6C F1 36 57 F6
PCR-05: 38 70 21 67 DB 54 96 54 A1 4F 45 5F 6A 32 42 EF EC 51 21 F5
PCR-06: 17 74 56 21 A9 45 7A 43 5C AD 2E 9E 96 4C EE 6B 6C EC FA 25
PCR-07: E3 0D 10 07 E5 38 19 5D 25 1E 8E 49 6E DE BF 8F AE 38 20 21
PCR-08: B9 1D 40 71 B0 AB AF 01 BD 14 1D 2B 7C 5B AF 66 9A B7 2C 00
PCR-09: D3 D4 51 B9 CA 9D FE 28 DC 5E AD 02 9A 84 44 67 49 48 0A 87
PCR-10: 6A 30 46 F0 4E DC D3 A8 A5 4F 4C 26 0F 64 63 0C 83 83 C7 3A
PCR-11: 42 5D 51 0A 0B 91 4C A3 1F 76 26 98 A8 97 8C 32 46 A0 92 6F
PCR-12: BD 7D 9D 93 C7 B2 17 80 38 E3 55 E9 45 19 3B 55 0A 3F EF 06
PCR-13: 39 0B 31 0A 42 EC 07 07 A2 02 E5 A6 D3 CB 8E BB 33 FD 7C 0D
PCR-14: 98 BB 81 70 A6 F3 7B 3A 4B 79 45 C0 15 2F DC EE 5F A1 1F 3B
PCR-15: 06 86 9D E0 B9 0E 0E D6 12 37 5C 9C 68 74 67 D2 7E 47 7B D4
PCR-16: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-17: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-18: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-19: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-21: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-22: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
PCR-23: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

# ./tpm_extendpcr -i pcr.txt -p 0
# cat /sys/class/misc/tpm0/device/pcrs

PCR-00: 9A D9 2D 86 9D 81 BD 58 08 7C F7 8E C6 31 CB BF 0C 9D 0D 28
PCR-01: 49 20 44 4B 1E AF B2 AA 4A C1 2B D1 44 2B 82 1F 52 EC E7 4B
PCR-02: 38 53 A8 EF 61 83 59 ED 7F 7F 2E CC 7B C8 D2 F3 87 EB 7C 55
PCR-03: 2C 2F B4 2A 15 36 B2 28 C6 01 40 D8 64 D7 30 7F AA 6D 91 54
PCR-04: 2E CF 07 F9 C7 30 B4 4C EE 19 7B 0D 36 4E EE 6C F1 36 57 F6
PCR-05: 38 70 21 67 DB 54 96 54 A1 4F 45 5F 6A 32 42 EF EC 51 21 F5
PCR-06: 17 74 56 21 A9 45 7A 43 5C AD 2E 9E 96 4C EE 6B 6C EC FA 25
PCR-07: E3 0D 10 07 E5 38 19 5D 25 1E 8E 49 6E DE BF 8F AE 38 20 21
PCR-08: B9 1D 40 71 B0 AB AF 01 BD 14 1D 2B 7C 5B AF 66 9A B7 2C 00
PCR-09: D3 D4 51 B9 CA 9D FE 28 DC 5E AD 02 9A 84 44 67 49 48 0A 87
PCR-10: 6A 30 46 F0 4E DC D3 A8 A5 4F 4C 26 0F 64 63 0C 83 83 C7 3A
PCR-11: 42 5D 51 0A 0B 91 4C A3 1F 76 26 98 A8 97 8C 32 46 A0 92 6F
PCR-12: BD 7D 9D 93 C7 B2 17 80 38 E3 55 E9 45 19 3B 55 0A 3F EF 06
PCR-13: 39 0B 31 0A 42 EC 07 07 A2 02 E5 A6 D3 CB 8E BB 33 FD 7C 0D
PCR-14: 98 BB 81 70 A6 F3 7B 3A 4B 79 45 C0 15 2F DC EE 5F A1 1F 3B
PCR-15: 06 86 9D E0 B9 0E 0E D6 12 37 5C 9C 68 74 67 D2 7E 47 7B D4
PCR-16: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-17: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-18: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-19: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-21: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-22: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
PCR-23: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

# reboot
# cat /sys/class/misc/tpm0/device/pcrs

PCR-00: 9A D9 2D 86 9D 81 BD 58 08 7C F7 8E C6 31 CB BF 0C 9D 0D 28
PCR-01: 2E 0D 03 0E 76 63 BE 09 DC 86 E8 1F 54 C2 3E 7C C7 C6 AD 9D
PCR-02: 4A A6 1A 10 8B 42 18 05 C9 61 E7 CD 1C BD 6A E9 02 F3 CC E0
PCR-03: 3A E2 33 E3 2D 76 3C A6 0D 40 BB 50 AC 28 20 CC A4 57 63 43
PCR-04: 60 19 D2 55 90 F7 D4 69 01 F2 18 1A AD 54 5A 77 11 CE 28 9E
PCR-05: 03 24 C9 EE A6 AE 65 65 51 1A 5B F2 68 2B C0 0F 56 48 80 31
PCR-06: 17 74 56 21 A9 45 7A 43 5C AD 2E 9E 96 4C EE 6B 6C EC FA 25
PCR-07: E3 0D 10 07 E5 38 19 5D 25 1E 8E 49 6E DE BF 8F AE 38 20 21
PCR-08: B9 1D 40 71 B0 AB AF 01 BD 14 1D 2B 7C 5B AF 66 9A B7 2C 00
PCR-09: D3 D4 51 B9 CA 9D FE 28 DC 5E AD 02 9A 84 44 67 49 48 0A 87
PCR-10: 6A 30 46 F0 4E DC D3 A8 A5 4F 4C 26 0F 64 63 0C 83 83 C7 3A
PCR-11: 42 5D 51 0A 0B 91 4C A3 1F 76 26 98 A8 97 8C 32 46 A0 92 6F
PCR-12: BD 7D 9D 93 C7 B2 17 80 38 E3 55 E9 45 19 3B 55 0A 3F EF 06
PCR-13: 39 0B 31 0A 42 EC 07 07 A2 02 E5 A6 D3 CB 8E BB 33 FD 7C 0D
PCR-14: 98 BB 81 70 A6 F3 7B 3A 4B 79 45 C0 15 2F DC EE 5F A1 1F 3B
PCR-15: 06 86 9D E0 B9 0E 0E D6 12 37 5C 9C 68 74 67 D2 7E 47 7B D4
PCR-16: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-17: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-18: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-19: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-21: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
PCR-22: FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF
PCR-23: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

How do I reset the PCR0? I'm not able to understand why the values of PCRs 1-15 also got modified. I also tried extending on the PCR16 and it also didn't get reset on reboot.

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  • 1
    Do the PCRs get cleared when the hardware looses power? Mar 3, 2017 at 10:10
  • I had also extended PCR-16. When the hardware lost power, PCR-16 got reseted to all zeros but PCR 0-15 did not. Mar 3, 2017 at 11:20
  • 1
    Does your embedded board come with any documentation about this? It could be a configurable option in the firmware. Mar 3, 2017 at 11:52
  • There's no documentation about this. I've checked. Mar 3, 2017 at 15:13
  • Seems like your firmware is not properly resetting the Static Root of Trust for Measurement (SRTM). Typically, this is a signed blob or a read only portion of your firmware. I suggest you check your BIOS options and/or your BIOS as a problem and should be updated.
    – northox
    Apr 24, 2017 at 21:19

2 Answers 2

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The only way to add data to a PCR is with TPM Extend Current value of a PCR is X. (Say, 0x0000....0000.) We extend the PCR with some data Y. Y must be 160 bit (20 byte) value 20 bytes = SHA1 hash, allowing longer data TPM calculates hash(Y,X)=Z; changes value in PCR to Z. We can update further: Extend with A: value is hash(A,Z)=hash(A, hash(Y,X)) Extend with B: PCR value is hash(B, hash(A,Z)) ...etc. Verifiers who know the values extended into the PCRs can easily verify Perform the same hash chain themselves Computationally infeasible to forge (must break SHA-1) Given PCR state N and desired state M, adversary would need to find X such that hash(X,N)=M; violates one-way assumption

Some (but not all) PCRs are resettable. This means they can be reset to a known state by executing the TPM PCR Reset command. Whether a given PCR is resettable or not is defined in platform spec All PC client TPMs have the same settings Server or virtual TPMs could differ; specs do not exist yet Reset requires appropriate permissions Usually based on locality, which we’ll discuss next Sets PCR value back to default, erasing all data currently present Either 0x000...000 or 0xFFF...FFF, depending on PCR & machine state

SOURCE

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    If quoting someone else's work, please cite your source instead of passing it off as your own.
    – schroeder
    Mar 3, 2017 at 15:42
  • @MertYilmaz, I'm not trying to use the TPM_PCR Reset command. PCR0-15 were not reset even after reboot. I just cannot understand this behavior. Mar 6, 2017 at 5:25
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PCR0 may have already been extended by the SRTM (e.g. BootROM) on early boot, so by the time you get to read it (user space), it's already been extended. Similar for PCR1-15. See section 3.3.4 and respective sub-sections of this document for the way PCs do it: https://trustedcomputinggroup.org/wp-content/uploads/TCG-PC-Client-Platform-Firmware-Profile-Version-1.06-Revision-52_pub-1.pdf

Some embedded platforms may follow the PC approach.

To play with PCRs in isolation, a software TPM may be a better choice: https://github.com/microsoft/ms-tpm-20-ref https://github.com/stefanberger/swtpm

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  • Now I notice this was a 6 year old question, so not sure why it appeared in the top questions list...
    – Silviu
    Feb 8 at 15:37
  • 2
    Questions without answers or with unvoted answers get bumped to the top for visibility.
    – schroeder
    Feb 8 at 15:59

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