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

Roberto Sassu
2017-05-30T07:31:02Z

On 5/30/2017 5:29 AM, Mimi Zohar wrote:
> 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.

The alternative is to extend remaining banks with a digest,
for example the first passed by the caller. I will modify
the patch, if everyone agrees on that.

Roberto

>
> 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,
>