Re: [tpmdd-devel] [PATCH v3 3/6] tpm: introduce tpm_pcr_bank_info structure with digest_size from TPM

Mimi Zohar
2017-06-27T15:25:01Z

On Wed, 2017-06-21 at 16:29 +0200, Roberto Sassu wrote:
> This patch introduces the new structure tpm_pcr_bank_info to store
> information regarding PCR banks. The next patch will replace the array of
> TPM algorithms IDs with an array of the new structure.
>
> tpm_pcr_bank_info contains the TPM algorithm ID, the digest size and,
> optionally, the corresponding crypto ID, if a mapping exists. These
> information will be used by IMA to calculate the digest of an event
> and to provide measurements logs to userspace applications. The new
> structure has been defined in include/linux/tpm.h, as it will be passed
> to functions outside the TPM driver.
>
> The purpose of this patch is to fix a serious issue in tpm2_pcr_extend():
> if the mapping between a TPM algorithm and a crypto algorithm is not
> defined, the PCR bank with the unknown algorithm is not extended.
> This gives the opportunity to an attacker to reply to remote attestation
> requests with a list of fake measurements. Instead, the digest size
> is retrieved from the output buffer of a PCR read, without relying
> on the crypto subsystem.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>

To address some of Jarkko's comments, I would replace active_banks
with a structure, named for example active_bank_info, containing the
existing active_bank alg_id and just the digest size.  A subsequent
patch would include the crypto_id, assuming it is needed.

When a patch defines a new field/function, at least one usage of that
field/function should be included in the patch.  In this case, you
might want to verify the digest size returned by the TPM is as
expected, comparing it with the crypto digest size.

Mimi

> ---
> drivers/char/tpm/tpm.h | 11 -----------
> drivers/char/tpm/tpm2-cmd.c | 30 ++++++++++++++++++++++++++++++
> include/linux/tpm.h | 19 +++++++++++++++++++
> 3 files changed, 49 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> index 1df0521..62c600d 100644
> --- a/drivers/char/tpm/tpm.h
> +++ b/drivers/char/tpm/tpm.h
> @@ -98,17 +98,6 @@ enum tpm2_return_codes {
> TPM2_RC_REFERENCE_H0 = 0x0910,
> };
>
> -enum tpm2_algorithms {
> - TPM2_ALG_ERROR = 0x0000,
> - TPM2_ALG_SHA1 = 0x0004,
> - TPM2_ALG_KEYEDHASH = 0x0008,
> - TPM2_ALG_SHA256 = 0x000B,
> - TPM2_ALG_SHA384 = 0x000C,
> - TPM2_ALG_SHA512 = 0x000D,
> - TPM2_ALG_NULL = 0x0010,
> - TPM2_ALG_SM3_256 = 0x0012,
> -};
> -
> enum tpm2_command_codes {
> TPM2_CC_FIRST = 0x011F,
> TPM2_CC_SELF_TEST = 0x0143,
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 6a9fe0d..74a68ea 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -992,6 +992,36 @@ int tpm2_probe(struct tpm_chip *chip)
> }
> EXPORT_SYMBOL_GPL(tpm2_probe);
>
> +static int tpm2_init_pcr_bank_info(struct tpm_chip *chip, u16 alg_id,
> + struct tpm_pcr_bank_info *active_bank)
> +{
> + struct tpm_buf buf;
> + struct tpm2_pcr_read_out *pcrread_out;
> + int rc = 0;
> + int i;
> +
> + active_bank->alg_id = alg_id;
> +
> + rc = tpm2_pcr_read_tpm_buf(chip, 0, alg_id, &buf, NULL);
> + if (rc)
> + goto out;
> +
> + pcrread_out = (struct tpm2_pcr_read_out *)&buf.data[TPM_HEADER_SIZE];
> +
> + active_bank->digest_size = be16_to_cpu(pcrread_out->digest_size);
> + active_bank->crypto_id = HASH_ALGO__LAST;
> +
> + for (i = 0; i < ARRAY_SIZE(tpm2_hash_map); i++) {
> + if (active_bank->alg_id != tpm2_hash_map[i].tpm_id)
> + continue;
> +
> + active_bank->crypto_id = tpm2_hash_map[i].crypto_id;
> + }
> +out:
> + tpm_buf_destroy(&buf);
> + return rc;
> +}
> +
> struct tpm2_pcr_selection {
> __be16 hash_alg;
> u8 size_of_select;
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 5a090f5..ff06738 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -22,6 +22,8 @@
> #ifndef __LINUX_TPM_H__
> #define __LINUX_TPM_H__
>
> +#include <crypto/hash_info.h>
> +
> #define TPM_DIGEST_SIZE 20 /* Max TPM v1.2 PCR size */
>
> /*
> @@ -37,6 +39,17 @@ enum TPM_OPS_FLAGS {
> TPM_OPS_AUTO_STARTUP = BIT(0),
> };
>
> +enum tpm2_algorithms {
> + TPM2_ALG_ERROR = 0x0000,
> + TPM2_ALG_SHA1 = 0x0004,
> + TPM2_ALG_KEYEDHASH = 0x0008,
> + TPM2_ALG_SHA256 = 0x000B,
> + TPM2_ALG_SHA384 = 0x000C,
> + TPM2_ALG_SHA512 = 0x000D,
> + TPM2_ALG_NULL = 0x0010,
> + TPM2_ALG_SM3_256 = 0x0012,
> +};
> +
> struct tpm_class_ops {
> unsigned int flags;
> const u8 req_complete_mask;
> @@ -52,6 +65,12 @@ struct tpm_class_ops {
> void (*relinquish_locality)(struct tpm_chip *chip, int loc);
> };
>
> +struct tpm_pcr_bank_info {
> + enum tpm2_algorithms alg_id;
> + enum hash_algo crypto_id;
> + u32 digest_size;
> +};
> +
> #if defined(CONFIG_TCG_TPM) || defined(CONFIG_TCG_TPM_MODULE)
>
> extern int tpm_is_tpm2(u32 chip_num);