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

[Jason Andryuk <jandryuk@gmail.com>]

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