Re: [PATCH] KEYS: trusted: allow module init if TPM is inactive or deactivated

[Tyler Hicks <tyhicks@canonical.com>]

On 2019-07-05 18:37:35, Roberto Sassu wrote:
> Commit c78719203fc6 ("KEYS: trusted: allow trusted.ko to initialize w/o a
> TPM") allows the trusted module to be loaded even a TPM is not found to
> avoid module dependency problems.
> 
> Unfortunately, this does not completely solve the issue, as there could be
> a case where a TPM is found but is not functional (the TPM commands return
> an error). Specifically, after the tpm_chip structure is returned by
> tpm_default_chip() in init_trusted(), the execution terminates after
> init_digests() returns -EFAULT (due to the fact that tpm_get_random()
> returns a positive value, but less than TPM_MAX_DIGEST_SIZE).
> 
> This patch fixes the issue by ignoring the TPM_ERR_DEACTIVATED and
> TPM_ERR_DISABLED errors.
> 
> Fixes: 240730437deb ("KEYS: trusted: explicitly use tpm_chip structure...")
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
>  drivers/char/tpm/tpm.h  | 2 --
>  include/linux/tpm.h     | 3 +++
>  security/keys/trusted.c | 6 +++++-
>  3 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> index e503ffc3aa39..a216ac396711 100644
> --- a/drivers/char/tpm/tpm.h
> +++ b/drivers/char/tpm/tpm.h
> @@ -54,8 +54,6 @@ enum tpm_addr {
>  
>  #define TPM_WARN_RETRY          0x800
>  #define TPM_WARN_DOING_SELFTEST 0x802
> -#define TPM_ERR_DEACTIVATED     0x6
> -#define TPM_ERR_DISABLED        0x7
>  #define TPM_ERR_INVALID_POSTINIT 38
>  
>  #define TPM_HEADER_SIZE		10
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 53c0ea9ec9df..efd3ccbb6aee 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -26,6 +26,9 @@
>  #define TPM_DIGEST_SIZE 20	/* Max TPM v1.2 PCR size */
>  #define TPM_MAX_DIGEST_SIZE SHA512_DIGEST_SIZE
>  
> +#define TPM_ERR_DEACTIVATED     0x6
> +#define TPM_ERR_DISABLED        0x7
> +
>  struct tpm_chip;
>  struct trusted_key_payload;
>  struct trusted_key_options;
> diff --git a/security/keys/trusted.c b/security/keys/trusted.c
> index 9a94672e7adc..430d85090b3b 100644
> --- a/security/keys/trusted.c
> +++ b/security/keys/trusted.c
> @@ -389,6 +389,10 @@ static int pcrlock(const int pcrnum)
>  	if (!capable(CAP_SYS_ADMIN))
>  		return -EPERM;
>  
> +	/* This can happen if the TPM is inactive. */
> +	if (!digests)
> +		return -EINVAL;
> +
>  	return tpm_pcr_extend(chip, pcrnum, digests) ? -EINVAL : 0;
>  }
>  
> @@ -1233,7 +1237,7 @@ static int __init init_digests(void)
>  	int i;
>  
>  	ret = tpm_get_random(chip, digest, TPM_MAX_DIGEST_SIZE);
> -	if (ret < 0)
> +	if (ret < 0 || ret == TPM_ERR_DEACTIVATED || ret == TPM_ERR_DISABLED)
>  		return ret;

As someone who hasn't looked at much of the TPM code, I would have
expected tpm_get_random() to return a positive value that only ever
indicates the number of random bytes saved to the buffer. From the
function documentation:

  Return: number of random bytes read or a negative error value.

Despite the function documentation and as your patch suggests, I can
see that it is possible for tpm_transmit_cmd() to return
a positive value that's also returned by tpm_get_random() even though it
may not have filled the buffer when the TPM is in an
inactive/deactivated state.

I think there are other callers which are not prepared for positive
return values that indicate a failure to fill the buffer with random
data. For instance, the way that tpm_hwrng_read() is calling
tpm_get_random() looks a little worrisome.

This patch would likely fix the bug reported against eCryptfs
(https://bugzilla.kernel.org/show_bug.cgi?id=203953) but I can't help to
think that callers of tpm_get_random() would benefit from a more
consolidated approach of handling TPM_ERR_* return values rather than
handling them at this single call site.

Tyler

>  	if (ret < TPM_MAX_DIGEST_SIZE)
>  		return -EFAULT;
> -- 
> 2.17.1
>