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Date:   Tue, 24 Nov 2020 10:14:22 -0300
From:   Ezequiel Garcia <ezequiel@...labora.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>,
        Adrian Ratiu <adrian.ratiu@...labora.com>
Cc:     linux-integrity@...r.kernel.org, Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Helen Koike <helen.koike@...labora.com>,
        Duncan Laurie <dlaurie@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>, kernel@...labora.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] char: tpm: add i2c driver for cr50

Hi Jarkko,

Thanks for your review.

On Tue, 2020-11-24 at 00:06 +0200, Jarkko Sakkinen wrote:
> On Fri, Nov 20, 2020 at 07:23:45PM +0200, Adrian Ratiu wrote:
> > From: "dlaurie@...omium.org" <dlaurie@...omium.org>
> > 
> > Add TPM 2.0 compatible I2C interface for chips with cr50 firmware.
> > 
> > The firmware running on the currently supported H1 MCU requires a
> > special driver to handle its specific protocol, and this makes it
> > unsuitable to use tpm_tis_core_* and instead it must implement the
> > underlying TPM protocol similar to the other I2C TPM drivers.
> > 
> > - All 4 byes of status register must be read/written at once.
> > - FIFO and burst count is limited to 63 and must be drained by AP.
> > - Provides an interrupt to indicate when read response data is ready
> > and when the TPM is finished processing write data.
> > 
> > This driver is based on the existing infineon I2C TPM driver, which
> > most closely matches the cr50 i2c protocol behavior.
> > 
> > Cc: Helen Koike <helen.koike@...labora.com>
> > Signed-off-by: Duncan Laurie <dlaurie@...omium.org>
> > [swboyd@...omium.org: Depend on i2c even if it's a module, replace
> > boilier plate with SPDX tag, drop asm/byteorder.h include, simplify
> > return from probe]
> > Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> > Signed-off-by: Fabien Lahoudere <fabien.lahoudere@...labora.com>
> > Signed-off-by: Adrian Ratiu <adrian.ratiu@...labora.com>
> > ---
> > Changes in v2:
> >   - Various small fixes all over (reorder includes, MAX_BUFSIZE, comments, etc)
> >   - Reworked return values of i2c_wait_tpm_ready() to fix timeout mis-handling
> > so ret == 0 now means success, the wait period jiffies is ignored because that
> > number is meaningless and return a proper timeout error in case jiffies == 0.
> >   - Make i2c default to 1 message per transfer (requested by Helen)
> >   - Move -EIO error reporting to transfer function to cleanup transfer() itself
> > and its R/W callers
> >   - Remove magic value hardcodings and introduce enum force_release.
> > 
> > v1 posted at https://lkml.org/lkml/2020/2/25/349
> > 
> > Applies on next-20201120, tested on Chromebook EVE.
> > ---
> >  drivers/char/tpm/Kconfig            |  10 +
> >  drivers/char/tpm/Makefile           |   2 +
> >  drivers/char/tpm/tpm_tis_i2c_cr50.c | 768 ++++++++++++++++++++++++++++
> >  3 files changed, 780 insertions(+)
> >  create mode 100644 drivers/char/tpm/tpm_tis_i2c_cr50.c
> > 
> > diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
> > index a18c314da211..4308f9ca7a43 100644
> > --- a/drivers/char/tpm/Kconfig
> > +++ b/drivers/char/tpm/Kconfig
> > @@ -86,6 +86,16 @@ config TCG_TIS_SYNQUACER
> >  	  To compile this driver as a module, choose  M here;
> >  	  the module will be called tpm_tis_synquacer.
> >  
> > +config TCG_TIS_I2C_CR50
> > +	tristate "TPM Interface Specification 2.0 Interface (I2C - CR50)"
> > +	depends on I2C
> > +	select TCG_CR50
> > +	help
> > +	  This is a driver for the Google cr50 I2C TPM interface which is a
> > +	  custom microcontroller and requires a custom i2c protocol interface
> > +	  to handle the limitations of the hardware.  To compile this driver
> > +	  as a module, choose M here; the module will be called tcg_tis_i2c_cr50.
> > +
> >  config TCG_TIS_I2C_ATMEL
> >  	tristate "TPM Interface Specification 1.2 Interface (I2C - Atmel)"
> >  	depends on I2C
> > diff --git a/drivers/char/tpm/Makefile b/drivers/char/tpm/Makefile
> > index 84db4fb3a9c9..66d39ea6bd10 100644
> > --- a/drivers/char/tpm/Makefile
> > +++ b/drivers/char/tpm/Makefile
> > @@ -27,6 +27,8 @@ obj-$(CONFIG_TCG_TIS_SPI) += tpm_tis_spi.o
> >  tpm_tis_spi-y := tpm_tis_spi_main.o
> >  tpm_tis_spi-$(CONFIG_TCG_TIS_SPI_CR50) += tpm_tis_spi_cr50.o
> >  
> > +obj-$(CONFIG_TCG_TIS_I2C_CR50) += tpm_tis_i2c_cr50.o
> > +
> >  obj-$(CONFIG_TCG_TIS_I2C_ATMEL) += tpm_i2c_atmel.o
> >  obj-$(CONFIG_TCG_TIS_I2C_INFINEON) += tpm_i2c_infineon.o
> >  obj-$(CONFIG_TCG_TIS_I2C_NUVOTON) += tpm_i2c_nuvoton.o
> > diff --git a/drivers/char/tpm/tpm_tis_i2c_cr50.c b/drivers/char/tpm/tpm_tis_i2c_cr50.c
> > new file mode 100644
> > index 000000000000..37555dafdca0
> > --- /dev/null
> > +++ b/drivers/char/tpm/tpm_tis_i2c_cr50.c
> > @@ -0,0 +1,768 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Copyright 2016 Google Inc.
> > + *
> > + * Based on Linux Kernel TPM driver by
> > + * Peter Huewe <peter.huewe@...ineon.com>
> > + * Copyright (C) 2011 Infineon Technologies
> > + */
> > +
> > +/*
> > + * cr50 is a firmware for H1 secure modules that requires special
> > + * handling for the I2C interface.
> > + *
> > + * - Use an interrupt for transaction status instead of hardcoded delays
> > + * - Must use write+wait+read read protocol
> > + * - All 4 bytes of status register must be read/written at once
> > + * - Burst count max is 63 bytes, and burst count behaves
> > + *   slightly differently than other I2C TPMs
> > + * - When reading from FIFO the full burstcnt must be read
> > + *   instead of just reading header and determining the remainder
> > + */
> > +
> > +#include <linux/acpi.h>
> > +#include <linux/completion.h>
> > +#include <linux/i2c.h>
> > +#include <linux/interrupt.h>
> > +#include <linux/module.h>
> > +#include <linux/pm.h>
> > +#include <linux/slab.h>
> > +#include <linux/wait.h>
> > +
> > +#include "tpm_tis_core.h"
> > +
> > +#define CR50_MAX_BUFSIZE	64
> > +#define CR50_TIMEOUT_SHORT_MS	2	/* Short timeout during transactions */
> > +#define CR50_TIMEOUT_NOIRQ_MS	20	/* Timeout for TPM ready without IRQ */
> > +#define CR50_I2C_DID_VID	0x00281ae0L
> > +#define CR50_I2C_MAX_RETRIES	3	/* Max retries due to I2C errors */
> > +#define CR50_I2C_RETRY_DELAY_LO	55	/* Min usecs between retries on I2C */
> > +#define CR50_I2C_RETRY_DELAY_HI	65	/* Max usecs between retries on I2C */
> 
> CR50_ -> TPM_CR50_
> 
> > +
> > +#define TPM_I2C_ACCESS(l)	(0x0000 | ((l) << 4))
> > +#define TPM_I2C_STS(l)		(0x0001 | ((l) << 4))
> > +#define TPM_I2C_DATA_FIFO(l)	(0x0005 | ((l) << 4))
> > +#define TPM_I2C_DID_VID(l)	(0x0006 | ((l) << 4))
> > +
> > +struct priv_data {
> > +	int irq;
> > +	int locality;
> > +	struct completion tpm_ready;
> > +	u8 buf[CR50_MAX_BUFSIZE];
> > +};
> 
> tpm_i2c_cr50_priv_data
> 
> > +
> > +enum force_release {
> > +	CR50_NO_FORCE = 0x0,
> > +	CR50_FORCE = 0x1,
> > +};
> 
> I'd just 
> 
> #define TPM_I2C_CR50_NO_FORCE	0
> #define TPM_I2C_CR50_FORCE	1
> 

A proper enumerated type has advantages over a preprocessor macro:
even if the compiler won't warn you, static analyzers can warn
about a misuse.

Also, it allows for a more obvious prototype.

I am curious why do you propose this change?

Thanks,
Ezequiel

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