Date: Mon, 27 May 2024 18:12:02 +0300
From: "Jarkko Sakkinen" <jarkko@...nel.org>
To: "Jarkko Sakkinen" <jarkko@...nel.org>, "Vitor Soares"
 <ivitro@...il.com>, "James Bottomley"
 <James.Bottomley@...senPartnership.com>, <linux-integrity@...r.kernel.org>
Cc: <keyrings@...r.kernel.org>, "Peter Huewe" <peterhuewe@....de>, "Jason
 Gunthorpe" <jgg@...pe.ca>, "Mimi Zohar" <zohar@...ux.ibm.com>, "David
 Howells" <dhowells@...hat.com>, "Paul Moore" <paul@...l-moore.com>, "James
 Morris" <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>,
 <linux-kernel@...r.kernel.org>, <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 1/3] tpm: Disable TCG_TPM2_HMAC by default

On Mon May 27, 2024 at 6:01 PM EEST, Jarkko Sakkinen wrote:
> On Mon May 27, 2024 at 5:51 PM EEST, Jarkko Sakkinen wrote:
> > On Thu May 23, 2024 at 10:59 AM EEST, Vitor Soares wrote:
> > > On Wed, 2024-05-22 at 19:11 +0300, Jarkko Sakkinen wrote:
> > > > On Wed May 22, 2024 at 5:58 PM EEST, Vitor Soares wrote:
> > > > > I did run with ftrace, but need some more time to go through it.
> > > > > 
> > > > > Here the step I did:
> > > > > kernel config:
> > > > >   CONFIG_FUNCTION_TRACER
> > > > >   CONFIG_FUNCTION_GRAPH_TRACER
> > > > > 
> > > > > ftrace:
> > > > >   # set filters
> > > > >   echo tpm* > set_ftrace_filter
> > > > > 
> > > > >   # set tracer
> > > > >   echo function_graph > current_tracer
> > > > > 
> > > > >   # take the sample
> > > > >   echo 1 > tracing_on; time modprobe tpm_tis_spi; echo 0 > tracing_on
> > > > > 
> > > > > regards,
> > > > > Vitor Soares
> > > > 
> > > > I'm now compiling distro kernel (OpenSUSE) for NUC7 with v6.10 contents.
> > > > 
> > > > After I have that setup, I'll develop a perf test either with perf or
> > > > bpftrace. I'll come back with the possible CONFIG_* that should be in
> > > > place in your kernel. Might take up until next week as I have some
> > > > conference stuff to prepare but I try to have stuff ready early next
> > > > week.
> > > > 
> > > > No need to rush with this as long as possible patches go to rc2 or rc3.
> > > > Let's do a proper analysis instead.
> > > > 
> > > > In the meantime you could check if you get perf and/or bpftrace to 
> > > > your image that use to boot up your device. Preferably both but
> > > > please inform about this.
> > > > 
> > >
> > > I already have perf running, for the bpftrace I might not be able to help.
> >
> > The interesting function to look at with/without hmac is probably
> > tpm2_get_random().
> >
> > I attached a patch that removes hmac shenigans out of tpm2_get_random()
> > for the sake of proper comparative testing.
>
> Other thing that we need to measure is to split the cost into
> two parts:
>
> 1. Handshake, i.e. setting up and shutdowning a session.
> 2. Transaction, payload TPM command.
>
> This could be done by setting up couple of kprobes_events:
>
>   payload_event: tpm2_get_random() etc.
>   hmac_event: tpm2_start_auth_session(), tpm2_end_auth_session() etc.
>
> And just summing up the time for a boot to get a cost for hmac.
>
> I'd use bootconfig for this:
>
> https://www.kernel.org/doc/html/v6.9/trace/boottime-trace.html
>
> So I've made up plans how measure the incident but not sure when I
> have time to pro-actively work on a benchmark (thus sharing details).
>
> So I think with just proper bootconfig wtih no other tools uses this
> can be measured.


I'll disable this for anything else than X86_64 by default, and put
such patch to my next pull request.

Someone needs to do the perf analysis properly based on the above
descriptions. I cannot commit my time to promise them to get the
perf regressions fixed by time. I can only commit on limiting the
feature ;-)

It is thus better be conservative and reconsider opt-in post 6.10.
X86_64 is safeplay because even in that 2018 NUC7 based on Celeron,
hmac is just fine.

BR, Jarkko

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