Re: [RFC PATCH] tpm: Allow the TPM2 pcr_extend HMAC capability to be disabled on boot

Mimi Zohar
2024-11-07T01:07:51Z

On Thu, 2024-11-07 at 02:03 +0200, Jarkko Sakkinen wrote:
> On Thu Nov 7, 2024 at 1:52 AM EET, Mimi Zohar wrote:
> > On Thu, 2024-11-07 at 01:22 +0200, Jarkko Sakkinen wrote:
> > > On Thu Nov 7, 2024 at 12:52 AM EET, James Bottomley wrote:
> > > >
> > > > I'm a bit confused here. It's TPM2_PCR_Extend we have the trouble with
> > > > (as Mimi says in her email that you quoted) not TPM2_GetRandom.
> > > >
> > > > The random number generator reseed occurs in a kernel thread that fires
> > > > about once a minute, so it doesn't show up in really any of the boot
> > > > timings. Plus even with sessions added, what there now isn't a
> > > > significant overhead even to the running kernel given it's asynchronous
> > > > and called infrequently.
> > >
> > > Ah, right then we need the boot flag, and my earlier comments to the
> > > parameter apply. I've never used IMA so I don't actually even know in
> > > detail how it is using TPM.
> >
> > Huh? A simple explanation is that IMA-measurement maintains a measurement list,
> > similar to the pre-boot event log. Each IMA-measurement record extends the TPM
> > PCR (default PCR 10).
> >
> > Assuming IMA is enabled in the kernel, then just add "ima_policy=tcb" or
> > "ima_policy=critical_data" on the boot command line. To view the measurement
> > records, cat <securityfs>/integrity/ima/ascii_runtime_measurements. Normally
> > the IMA policy specified on the boot command line is replaced with a finer
> > grained custom policy.
>
> I'll try to figure out how to test it regularly. And yeah we need the
> flag obviously.
>
> I have my (CI compatible) framework that I run regularly with upstream
> that I've mentioned a few times earlier.
>
> https://codeberg.org/jarkko/linux-tpmdd-test
>
> How would I would make all files in /etc get to get the checksums, and
> how can I generate legit and illegit change to some file in that tree?
>
> No need to address how to implement that to my framework, I can figure
> that out. I just would love throw something so that any performance
> regressions will be catched right at the get go, i.e. before they
> end up to the mainline.

Yes, I still need to look at it. FYI, the IMA policy cannot be defined in terms
of pathnames. For testing, we've been loopback mounting a filesystem and
defining policy rules based on the UUID of the filesystem. If you're using
SELinux, then rules can be defined in terms of SELinux labels. There are other
methods of identifying files. Ken's been working on new IMA documentation[1],
which can be viewed here
https://ima-doc.readthedocs.io/en/latest/ima-concepts.html .

Here are some examples as to how to locally verify the IMA measurement list and
the boot aggregate.

1. To locally verify the IMA measurement list matches TPM PCR-10, use evmctl
(ima-evm-utils). For example,

a. An IMA measurement list without integrity violations
(/sys/kernel/security/ima/violations)

evmctl ima_measurement /sys/kernel/security/ima/binary_runtime_measurements

b. An IMA measurement list with integrity violations

evmctl ima_measurement --ignore-violations
/sys/kernel/security/ima/binary_runtime_measurements

2. To locally verify the 'boot_aggregate' record, the first record in the IMA
measurement list, use "evmctl ima_boot_aggregate -v" and compare the resulting
hash with the one in the boot_aggregate record.

[1] https://github.com/linux-integrity/ima-doc
[2] https://github.com/linux-integrity/ima-evm-utils/tree/next-testing/

Mimi