Re: ima pcr question

[Jerry Snitselaar <jsnitsel@redhat.com>]

On Mon Jan 13 20, Mimi Zohar wrote:
>On Mon, 2020-01-13 at 17:06 -0700, Jerry Snitselaar wrote:
>> We had a report of messages from ima saying "Error communicating with
>> TPM".  Looking into it a bit, it looks like with some Dell systems
>> (possibly others as well) in the bios they can set the hash algorithm
>> being used. In this case with that set to sha256 the messages
>> appear. Flipping the system to using sha1 makes them disappear.
>> Looking at the ima code, ima_calc_boot_aggregate_tfm hard codes using
>> sha1. Should that be changed to use whatever the default is in the
>> config, or possibly find out from the tpm what algorithm is being used?
>
>The ima-ng template contains two digests.  The first digest is the
>value being extended into the TPM, while the second digest is either
>the boot aggregate or file data hash.  It sounds like the problem is
>with the first digest.  Changing the boot-aggregate to use sha256
>might be a good idea, but probably won't fix the problem.
>
>Mimi
>

The error message is coming from ima_pcrread, and the tpm_digest that gets passed
by ima_calc_boot_aggregate_tfm to ima_pcrread is declared:

	struct tpm_digest d = { .alg_id = TPM_ALG_SHA1, .digest = {0} };

According to Dell their default BIOS setting is to use sha256. What they see with that
setting is:

[ 5.475036] ima: Error Communicating to TPM chip
[ 5.475083] tsc: Refined TSC clocksource calibration: 3311.999 MHz
[ 5.475092] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x2fbd936b72f, max_idle_ns: 440795283163 ns
[ 5.475118] ima: Error Communicating to TPM chip
[ 5.475165] ima: Error Communicating to TPM chip
[ 5.475235] clocksource: Switched to clocksource tsc
[ 5.475266] ima: Error Communicating to TPM chip
[ 5.475311] ima: Error Communicating to TPM chip
[ 5.475341] ima: Error Communicating to TPM chip
[ 5.475371] ima: Error Communicating to TPM chip
[ 5.475402] ima: Error Communicating to TPM chip
[ 5.489049] ima: No architecture policies found