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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

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?

James

>  	return status;
>  }