Re: [PATCH] tpm_tis: work around status register bug in STMicroelectronics TPM

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Fri, Aug 28, 2020 at 08:12:55PM -0400, Jason Andryuk wrote:
> On Fri, Aug 28, 2020 at 7:18 PM Jarkko Sakkinen
> <jarkko.sakkinen@linux.intel.com> wrote:
> >
> > On Thu, Aug 27, 2020 at 11:24:45AM -0400, Jason Andryuk wrote:
> > > James Bottomley wrote:
> > > >On Wed, 2020-04-15 at 15:45 -0700, Omar Sandoval wrote:
> > > >> From: Omar Sandoval <osandov@fb.com>
> > > >>
> > > >> We've encountered a particular model of STMicroelectronics TPM that
> > > >> transiently returns a bad value in the status register. This causes
> > > >> the kernel to believe that the TPM is ready to receive a command when
> > > >> it actually isn't, which in turn causes the send to time out in
> > > >> get_burstcount(). In testing, reading the status register one extra
> > > >> time convinces the TPM to return a valid value.
> > > >
> > > >Interesting, I've got a very early upgradeable nuvoton that seems to be
> > > >behaving like this.
> > > >
> > > >> Signed-off-by: Omar Sandoval <osandov@fb.com>
> > > >> ---
> > > >>  drivers/char/tpm/tpm_tis_core.c | 12 ++++++++++++
> > > >>  1 file changed, 12 insertions(+)
> > > >>
> > > >> diff --git a/drivers/char/tpm/tpm_tis_core.c
> > > >> b/drivers/char/tpm/tpm_tis_core.c
> > > >> index 27c6ca031e23..277a21027fc7 100644
> > > >> --- a/drivers/char/tpm/tpm_tis_core.c
> > > >> +++ b/drivers/char/tpm/tpm_tis_core.c
> > > >> @@ -238,6 +238,18 @@ static u8 tpm_tis_status(struct tpm_chip *chip)
> > > >>    rc = tpm_tis_read8(priv, TPM_STS(priv->locality), &status);
> > > >>    if (rc < 0)
> > > >>            return 0;
> > > >> +  /*
> > > >> +   * Some STMicroelectronics TPMs have a bug where the status
> > > >> register is
> > > >> +   * sometimes bogus (all 1s) if read immediately after the
> > > >> access
> > > >> +   * register is written to. Bits 0, 1, and 5 are always
> > > >> supposed to read
> > > >> +   * as 0, so this is clearly invalid. Reading the register a
> > > >> second time
> > > >> +   * returns a valid value.
> > > >> +   */
> > > >> +  if (unlikely(status == 0xff)) {
> > > >> +          rc = tpm_tis_read8(priv, TPM_STS(priv->locality),
> > > >> &status);
> > > >> +          if (rc < 0)
> > > >> +                  return 0;
> > > >> +  }
> > > >
> > > >You theorize that your case is fixed by the second read, but what if it
> > > >isn't and the second read also returns 0xff?  Shouldn't we have a line
> > > >here saying
> > > >
> > > >if (unlikely(status == 0xff))
> > > >     status = 0;
> > > >
> > > >So if we get a second 0xff we just pretend the thing isn't ready?
> > >
> > > Thanks for the fix, Omar!
> > >
> > > I tried the patch and it helps with STM TPM2 issues where commands fail
> > > with the kernel reporting:
> > > tpm tpm0: Unable to read burstcount
> > > tpm tpm0: tpm_try_transmit: send(): error -16
> > >
> > > My testing was with 5.4, and I'd like to see this CC-ed stable.
> > >
> > > When trying to diagnose the issue before finding this patch, I found it
> > > was timing sensitive.  I was seeing failures in the OpenXT installer.
> > > The system is basically idle when issuing TPM commands which frequently
> > > failed.  The same hardware booted into a Fedora Live USB image didn't
> > > have any TPM command failures.  One notable difference between the two
> > > is Fedora is CONFIG_PREEMPT=y and OpenXT is CONFIG_PREEMPT_NONE=y.
> > > Switching OpenXT to PREEMPT=y helped some, but there were still some
> > > issues with commands failing.  The second interesting thing was running tpm
> > > commands in OpenXT under trace-cmd let them succeed.  I guess that was enough
> > > to throw the timing off.
> > >
> > > Anyway, I'd like to see this patch applied, please.
> > >
> > > Thanks,
> > > Jason
> >
> > There was v2 sent after this:
> >
> > https://patchwork.kernel.org/patch/11492125/
> 
> Thanks!  That one didn't come up in a search for STM on lore.kernel.org.
> 
> > Unfortunately it lacks changelog. What was changed between v1 and v2?
> > Why v3 has not been sent yet? I see a discussion with no final
> > conclusion.
> 
> Looks like v2 added James's suggestion with a comment (sorry the
> formating is off):
> 
> + /*
> + * The status is somehow still bad. This hasn't been observed in
> + * practice, but clear it just in case so that it doesn't appear
> + * to be ready.
> + */
> + if (unlikely(status == 0xff))
> +         status = 0;
> 
> But, yes, the decision on the alternate approach is unresolved.
> 
> Thanks again,
> Jason

I'm happy to apply this patch as soon as there is either v3 or some
resolution to v2 discussion.

/Jarkko