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

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Date:   Tue, 12 Mar 2019 10:14:16 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Peter Hüwe <PeterHuewe@....de>
Cc:     Calvin Owens <calvinowens@...com>, 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 12:59 -0400, Mimi Zohar wrote:
> On Tue, 2019-03-12 at 07:42 -0700, James Bottomley wrote:
> > 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, I thought Peter sent you a tis "debug" tool to help you debug
> the problem you're seeing.  Whatever happened?

No, not seen one.  I have tried to debug the problem, but it's really
odd: my TPM is a polled nuvoton (so no irq line).  If you poll the data
ready bit on my TPM too often, it simply drops off the bus and every
TPM operation after that times out.  The only way to recover is to
reboot.

James