Message-Id: <D5FKM963NJ6O.3BGXETHW2FC5K@kernel.org>
Date: Thu, 07 Nov 2024 03:55:07 +0200
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Mimi Zohar" <zohar@...ux.ibm.com>, "James Bottomley"
 <James.Bottomley@...senPartnership.com>, <linux-integrity@...r.kernel.org>
Cc: <roberto.sassu@...wei.com>, <mapengyu@...il.com>, "Paul Moore"
 <paul@...l-moore.com>, <linux-kernel@...r.kernel.org>,
 <christian@...sel.eu>, "Ken Goldman" <kgold@...ux.ibm.com>
Subject: Re: [RFC PATCH] tpm: Allow the TPM2 pcr_extend HMAC capability to
 be disabled on boot

On Thu Nov 7, 2024 at 3:07 AM EET, Mimi Zohar wrote:
> 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.

Thanks! I write an issue based on this to my Codeberg repository, and
purge it once the time. I'll start by that and later on formalize
some commits or perhaps IMA specific buildroot config...

As far as the patch goes, I thought that I refine the patch myself, and
save everyone's time and nervers from unnecessary reviews rounds. It
does not make any radical changes to the approach.

See https://lore.kernel.org/linux-integrity/20241107004708.108667-1-jarkko@kernel.org/

I cannot take reviewed/tested-by's from any of the authors but if you
can check that it works for you I can surely send it Linus without
further tags than three SOB's :-) That said happy to get at least
tested-by from someone.

I'll send a PR to Linus as soon as possible.

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

BR, Jarkko

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