RE: [PATCH v2] tpm/tpm_crb: Avoid unaligned reads in crb_recv()

["Winkler, Tomas" <tomas.winkler@intel.com>]

> 
> The current approach to read first 6 bytes from the response and then tail of
> the response, can cause the 2nd memcpy_fromio() to do an unaligned read
> (e.g. read 32-bit word from address aligned to a 16-bits), depending on how
> memcpy_fromio() is implemented. If this happens, the read will fail and the
> memory controller will fill the read with 1's.
> 
> This was triggered by 170d13ca3a2f, which should be probably refined to check
> and react to the address alignment. Before that commit, on x86
> memcpy_fromio() turned out to be memcpy(). By a luck GCC has done the right
> thing (from tpm_crb's perspective) for us so far, but we should not rely on that.
> Thus, it makes sense to fix this also in tpm_crb, not least because the fix can be
> then backported to stable kernels and make them more robust when compiled
> in differing environments.
> 
> Cc: stable@vger.kernel.org
> Cc: James Morris <jmorris@namei.org>
> Cc: Tomas Winkler <tomas.winkler@intel.com>
> Cc: Jerry Snitselaar <jsnitsel@redhat.com>
> Fixes: 30fc8d138e91 ("tpm: TPM 2.0 CRB Interface")
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
After fixing the typos you can add my ack. 
Thanks
Tomas

> ---
> v2:
> * There was a trailing double colon in the end of the short summary.
> * Check requested and expected length against TPM_HEADER_SIZE.
> * Add some explanatory comments to crb_recv().
>  drivers/char/tpm/tpm_crb.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm_crb.c b/drivers/char/tpm/tpm_crb.c index
> 36952ef98f90..c084e61299aa 100644
> --- a/drivers/char/tpm/tpm_crb.c
> +++ b/drivers/char/tpm/tpm_crb.c
> @@ -287,19 +287,29 @@ static int crb_recv(struct tpm_chip *chip, u8 *buf,
> size_t count)
>  	struct crb_priv *priv = dev_get_drvdata(&chip->dev);
>  	unsigned int expected;
> 
> -	/* sanity check */
> -	if (count < 6)
> +	/* A sanity check that the upper layer wants to get at least the header
> +	 * as that is the minimum size for any TPM response.
> +	 */
> +	if (count < TPM_HEADER_SIZE)
>  		return -EIO;
> 
> +	/* If this bit is set, according to the spec, the TPM is in unrecovable
                                                                                                     ^^^ typo ^^^^
> +	 * condition.
> +	 */
>  	if (ioread32(&priv->regs_t->ctrl_sts) & CRB_CTRL_STS_ERROR)
>  		return -EIO;
> 
> -	memcpy_fromio(buf, priv->rsp, 6);
> -	expected = be32_to_cpup((__be32 *) &buf[2]);
> -	if (expected > count || expected < 6)
> +	/* Read 8 bytes (not just 6 bytes, which would cover ^^^ tag and^^^ the response
> length
> +	 * field ^^^s^^^) in order to make sure that the reminding memory accesses
                                                                             ^^^ remaining^^^  
> will
> +	 * be aligned.
> +	 */
> +	memcpy_fromio(buf, priv->rsp, 8);
> +
> +	expected = be32_to_cpup((__be32 *)&buf[2]);
> +	if (expected > count || expected < TPM_HEADER_SIZE)
>  		return -EIO;
> 
> -	memcpy_fromio(&buf[6], &priv->rsp[6], expected - 6);
> +	memcpy_fromio(&buf[8], &priv->rsp[8], expected - 8);
> 
>  	return expected;
>  }
> --
> 2.19.1