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Subject: Re: [PATCH 1/3] tpm: Disable TCG_TPM2_HMAC by default
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Jarkko Sakkinen <jarkko@kernel.org>, Vitor Soares <ivitro@gmail.com>, 
	linux-integrity@vger.kernel.org
Cc: keyrings@vger.kernel.org, Peter Huewe <peterhuewe@gmx.de>, Jason
 Gunthorpe <jgg@ziepe.ca>, Mimi Zohar <zohar@linux.ibm.com>, David Howells
 <dhowells@redhat.com>, Paul Moore <paul@paul-moore.com>, James Morris
 <jmorris@namei.org>, "Serge E. Hallyn" <serge@hallyn.com>, 
 linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org
Date: Mon, 27 May 2024 13:57:49 -0400
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On Mon, 2024-05-27 at 18:34 +0300, Jarkko Sakkinen wrote:
> On Mon May 27, 2024 at 6:12 PM EEST, Jarkko Sakkinen wrote:
> > 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.
> 
> While looking at code I started to wanted what was the reasoning
> for adding *undocumented* "TPM2_OA_TMPL" in include/linux/tpm.h.
> It should really be in tpm2-sessions.c and named something like
> TPM2_NULL_KEY_OA or similar.

Well, because you asked for it. I originally had all the flags spelled
out and I'm not a fan of this obscurity, but you have to do stuff like
this to get patches accepted:

https://lore.kernel.org/linux-integrity/CZCKTWU6ZCC9.2UTEQPEVICYHL@suppilovahvero/

James