linux-kernel - Re: [PATCH] tpm: Make timeout logic simpler and more robust

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Date:   Tue, 12 Mar 2019 07:42:46 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     Calvin Owens <calvinowens@...com>, Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] tpm: Make timeout logic simpler and more robust

On Tue, 2019-03-12 at 14:50 +0200, Jarkko Sakkinen wrote:
> On Mon, Mar 11, 2019 at 05:27:43PM -0700, James Bottomley wrote:
> > On Mon, 2019-03-11 at 16:54 -0700, Calvin Owens wrote:
> > > e're having lots of problems with TPM commands timing out, and
> > > we're seeing these problems across lots of different hardware
> > > (both v1/v2).
> > > 
> > > I instrumented the driver to collect latency data, but I wasn't
> > > able to find any specific timeout to fix: it seems like many of
> > > them are too aggressive. So I tried replacing all the timeout
> > > logic with a single universal long timeout, and found that makes
> > > our TPMs 100% reliable.
> > > 
> > > Given that this timeout logic is very complex, problematic, and
> > > appears to serve no real purpose, I propose simply deleting all
> > > of it.
> > 
> > "no real purpose" is a bit strong given that all these timeouts are
> > standards mandated.  The purpose stated by the standards is that
> > there needs to be a way of differentiating the TPM crashed from the
> > TPM is taking a very long time to respond.  For a normally
> > functioning TPM it looks complex and unnecessary, but for a
> > malfunctioning one it's a lifesaver.
> 
> Standards should be only followed when they make practical sense and
> ignored when not. The range is only up to 2s anyway.

I don't disagree ... and I'm certainly not going to defend the TCG
because I do think the complexity of some of its standards contributed
to the lack of use of TPM 1.2.

However, I am saying we should root cause this problem rather than take
a blind shot at the apparent timeout complexity.  My timeout
instability is definitely related to the polling adjustments, so it's
not unreasonable to think Facebooks might be as well.

James