Re: [Linux-ima-devel] [PATCH v2 3/5] tpm: pass multiple digests to tpm_pcr_extend()

Mimi Zohar
2017-05-30T03:30:05Z

On Fri, 2017-05-05 at 16:21 +0200, Roberto Sassu wrote:
> The tpm_pcr_extend() definition has been modified to take an array of
> tpm2_digest structures, and the size of the array as arguments.
>
> The function now checks if callers provided a digests for each active
> PCR bank (or a SHA1 digest for TPM 1.2), to follow the recomendation from
> the TCG specifications. See commit c1f92b4b04ad ("tpm: enhance TPM 2.0
> PCR extend to support multiple banks"). All banks should be extended
> because unused banks could be used by an attacker to hide the true
> integrity status of the platform.
>
> The only allowed exception to the rule above is to pass a SHA1 digest.
> It has been introduced to maintain compatibility with applications that
> expect to interact with a TPM 1.2, and provide only a SHA1 digest.
> In this case, the behavior of tpm_pcr_extend() is unchanged and
> remaining PCR banks are extended with that digest, padded with zeros.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
> v2
>
> - tpm_pcr_extend() arguments checked by tpm_pcr_check_input()
> - modified parameters of tpm_pcr_extend()
>
> drivers/char/tpm/tpm-interface.c | 76 +++++++++++++++++++++++++++++++++++++---
> drivers/char/tpm/tpm.h | 6 ----
> include/linux/tpm.h | 11 ++++--
> 3 files changed, 80 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
> index aac703e..4b08b02 100644
> --- a/drivers/char/tpm/tpm-interface.c
> +++ b/drivers/char/tpm/tpm-interface.c
> @@ -867,6 +867,55 @@ static int tpm1_pcr_extend(struct tpm_chip *chip, int pcr_idx, const u8 *hash,
> }
>
> /**
> + * tpm_pcr_check_input - check digests argument
> + *
> + * Return values:
> + * 1: input correct
> + * 0: fill digests with SHA1 digest padded with zeros
> + * -EINVAL: input incorrect
> + */
> +static int tpm_pcr_check_input(struct tpm_chip *chip, int count,
> + struct tpm2_digest *digests)
> +{
> + bool sha1_only;
> + int found = 0, not_found = 0;
> + int i, j;
> +
> + if (count <= 0 || digests == NULL)
> + return -EINVAL;
> +
> + sha1_only = (count == 1 && digests[0].alg_id == TPM2_ALG_SHA1);
> +
> + if (!(chip->flags & TPM_CHIP_FLAG_TPM2))
> + return sha1_only ? 1 : -EINVAL;
> +
> + if (sha1_only)
> + return 0;
> +
> + for (i = 0; i < ARRAY_SIZE(chip->active_banks) &&
> + chip->active_banks[i] != TPM2_ALG_ERROR; i++) {
> + for (j = 0; j < count; j++) {
> + if (digests[j].alg_id == chip->active_banks[i]) {
> + found++;
> + break;
> + }
> + }
> +
> + if (j == count) {
> + dev_dbg(&chip->dev, "%s: missing algorithm 0x%X\n",
> + __func__, chip->active_banks[i]);
> + not_found++;
> + }
> + }
> +
> + if (not_found == 0 && found != count)
> + dev_dbg(&chip->dev,
> + "%s: duplicate or unsupported algorithm\n", __func__);
> +
> + return (not_found == 0 && found == count) ? 1 : -EINVAL;
> +}
> +
> +/**
> * tpm_pcr_extend - extend pcr value with hash
> * @chip_num: tpm idx # or AN&
> * @pcr_idx: pcr idx to extend
> @@ -876,29 +925,46 @@ static int tpm1_pcr_extend(struct tpm_chip *chip, int pcr_idx, const u8 *hash,
> * isn't, protect against the chip disappearing, by incrementing
> * the module usage count.
> */
> -int tpm_pcr_extend(u32 chip_num, int pcr_idx, const u8 *hash)
> +int tpm_pcr_extend(u32 chip_num, int pcr_idx, int count,
> + struct tpm2_digest *digests)
> {
> int rc;
> struct tpm_chip *chip;
> struct tpm2_digest digest_list[ARRAY_SIZE(chip->active_banks)];
> - u32 count = 0;
> + struct tpm2_digest *digests_ptr = digests;
> + u32 filled_count = 0;
> + u8 *hash;
> int i;
>
> chip = tpm_chip_find_get(chip_num);
> if (chip == NULL)
> return -ENODEV;
>
> - if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> + rc = tpm_pcr_check_input(chip, count, digests);
> + if (rc < 0) {
> + dev_dbg(&chip->dev, "%s: invalid arguments\n", __func__);
> + tpm_put_ops(chip);

This rejects the TPM extend, if ANY of the algorithms are unknown.
Suppose that the standards were updated, TPM vendors add support for
the new algorithm, but the kernel has not been updated to reflect the
new algorithms supported.  As the measurement hash already been added
to the IMA measurement list, verifying the measurement list against a
TPM quote will fail, not just for the unknown algorithm, but for all
algorithms.  Something is very broken with this approach.

Mimi

> + return rc;
> + }
> +
> + hash = digests[0].digest;
> +
> + if (!rc) {
> memset(digest_list, 0, sizeof(digest_list));
>
> for (i = 0; i < ARRAY_SIZE(chip->active_banks) &&
> chip->active_banks[i] != TPM2_ALG_ERROR; i++) {
> digest_list[i].alg_id = chip->active_banks[i];
> memcpy(digest_list[i].digest, hash, TPM_DIGEST_SIZE);
> - count++;
> + filled_count++;
> }
>
> - rc = tpm2_pcr_extend(chip, pcr_idx, count, digest_list);
> + digests_ptr = digest_list;
> + count = filled_count;
> + }
> +
> + if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> + rc = tpm2_pcr_extend(chip, pcr_idx, count, digests_ptr);
> tpm_put_ops(chip);
> return rc;
> }
> diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> index b22bc25..6d775c4 100644
> --- a/drivers/char/tpm/tpm.h
> +++ b/drivers/char/tpm/tpm.h
> @@ -34,7 +34,6 @@
> #include <linux/acpi.h>
> #include <linux/cdev.h>
> #include <linux/highmem.h>
> -#include <crypto/hash_info.h>
>
> enum tpm_const {
> TPM_MINOR = 224, /* officially assigned */
> @@ -405,11 +404,6 @@ struct tpm_cmd_t {
> tpm_cmd_params params;
> } __packed;
>
> -struct tpm2_digest {
> - u16 alg_id;
> - u8 digest[SHA512_DIGEST_SIZE];
> -} __packed;
> -
> /* A string buffer type for constructing TPM commands. This is based on the
> * ideas of string buffer code in security/keys/trusted.h but is heap based
> * in order to keep the stack usage minimal.
> diff --git a/include/linux/tpm.h b/include/linux/tpm.h
> index 9ecd12c..5c5a600 100644
> --- a/include/linux/tpm.h
> +++ b/include/linux/tpm.h
> @@ -36,6 +36,11 @@ struct tpm_chip;
> struct trusted_key_payload;
> struct trusted_key_options;
>
> +struct tpm2_digest {
> + u16 alg_id;
> + u8 digest[SHA512_DIGEST_SIZE];
> +} __packed;
> +
> enum TPM_OPS_FLAGS {
> TPM_OPS_AUTO_STARTUP = BIT(0),
> };
> @@ -70,7 +75,8 @@ struct tpm_class_ops {
>
> extern int tpm_is_tpm2(u32 chip_num);
> extern int tpm_pcr_read(u32 chip_num, int pcr_idx, u8 *res_buf);
> -extern int tpm_pcr_extend(u32 chip_num, int pcr_idx, const u8 *hash);
> +extern int tpm_pcr_extend(u32 chip_num, int pcr_idx, int count,
> + struct tpm2_digest *digests);
> extern int tpm_pcr_algorithms(u32 chip_num, int count,
> enum tpm2_algorithms *algorithms);
> extern enum hash_algo tpm_pcr_algo_to_crypto(enum tpm2_algorithms tpm_id);
> @@ -91,7 +97,8 @@ static inline int tpm_is_tpm2(u32 chip_num)
> static inline int tpm_pcr_read(u32 chip_num, int pcr_idx, u8 *res_buf) {
> return -ENODEV;
> }
> -static inline int tpm_pcr_extend(u32 chip_num, int pcr_idx, const u8 *hash) {
> +static inline int tpm_pcr_extend(u32 chip_num, int pcr_idx, int count,
> + struct tpm2_digest *digests) {
> return -ENODEV;
> }
> static inline int tpm_pcr_algorithms(u32 chip_num, int count,