Re: [PATCH v3 0/6] Updated API for TPM 2.0 PCR extend

Roberto Sassu
2017-06-26T07:28:27Z

On 6/24/2017 11:03 AM, Jarkko Sakkinen wrote:
> On Wed, Jun 21, 2017 at 04:29:35PM +0200, Roberto Sassu wrote:
>> The first version of the patch set can be retrieved at the URL:
>>
>> https://sourceforge.net/p/tpmdd/mailman/message/35756302/
>>
>> The patches should be applied on top of the next branch of
>> linux-security.git (commit cdac74d).
>>
>> This patch set updates the TPM driver API for extending Platform
>> Configuration Registers (PCRs). These are special TPM registers which
>> cannot be written directly, but can only be updated through the extend
>> operation, by passing a digest as input:
>>
>> PCR_value_new = hash_func(PCR_value_old | digest)
>>
>> While TPM 1.2 can only use SHA1 as hash function, TPM 2.0 can support
>> multiple algorithms. On a TPM 2.0, PCR values extended with the same
>> algorithm are stored in a location called bank.
>>
>> Currently, PCRs can only be extended from the kernel with a SHA1 digest,
>> through tpm_pcr_extend(). Remaining banks of a TPM 2.0 are extended with
>> the SHA1 digest padded with zeros. In order to take advantage of stronger
>> algorithms, the TPM driver should allow TPM users to extend a PCR with
>> digests calculated with the same algorithm of the PCR bank.
>>
>> This patch set first introduces a new structure, called tpm_pcr_bank_info,
>> for storing detailed information about each PCR bank:
>>
>> - TPM algorithm ID: algorithm identifier, defined by TCG; will be included
>> by TPM users (e.g. IMA) in a measurement event log
>> - digest size: digest size for a TPM algorithm; since this is retrieved
>> from the TPM, PCR banks can be extended even if an algorithm is unknown
>> for the crypto subsystem (which currently the TPM driver relies on)
>> - crypto ID: will be used by TPM users to calculate a digest, to extend
>> a PCR
>>
>> Then, the patch set introduces the new function tpm_get_pcr_banks_info(),
>> to pass the information about PCR banks to TPM users, and finally, modifies
>> the parameters of tpm_pcr_extend(), so that TPM users can pass multiple
>> digests.
>>
>> Given this definition of the tpm2_digest structure:
>>
>> struct tpm2_digest {
>> u16 alg_id;
>> u8 digest[SHA512_DIGEST_SIZE];
>> } __packed;
>>
>> there will be two methods to extend PCRs:
>>
>> 1) by passing only one tpm2_digest structure containing a SHA1 digest
>> (as it is done in the patch 6/6); in this case, the SHA1 digest
>> is padded with zeros
>>
>> 2) by calling tpm_get_pcr_banks_info() to obtain PCR banks information,
>> and by calling tpm_pcr_extend() with as many tpm2_digest structures as
>> the number of tpm_pcr_bank_info structures retrieved in the first step
>> (if a digest for a TPM algorithm is missing, the TPM driver
>> pads/truncates the first digest provided by the TPM user)
>>
>>
>> API Usage Examples
>>
>> In the following examples, an application extends PCR 16 with the digest
>> of an event (e.g. record of a software measurement), with the methods
>> described above.
>>
>>
>> void app_calc_event_digest(struct crypto_shash *tfm, char *event,
>> u8 *digest)
>> {
>> SHASH_DESC_ON_STACK(shash, tfm);
>>
>> shash->tfm = tfm;
>> shash->flags = 0;
>>
>> crypto_shash_init(shash);
>> crypto_shash_update(shash, event, strlen(event));
>> crypto_shash_final(shash, digest);
>> }
>>
>> void app_pcr_extend_method_1(void)
>> {
>> char *event = "application event";
>> struct tpm2_digest digestarg = {.alg_id = TPM2_ALG_SHA1};
>>
>> /* calculate event digest with current hash algorithm */
>> struct crypto_shash *tfm = crypto_alloc_shash("sha1", 0, 0);
>>
>> app_calc_event_digest(tfm, event, digestarg.digest);
>>
>> /* extend all PCR banks with SHA1 digest*/
>> tpm_pcr_extend(TPM_ANY_NUM, 16, 1, &digestarg);
>> }
>>
>> void app_pcr_extend_method_2(void)
>> {
>> /* declare static arrays, limit is known */
>> struct tpm_pcr_bank_info active_banks[TPM_ACTIVE_BANKS_MAX];
>> struct tpm2_digest digest_array[TPM_ACTIVE_BANKS_MAX];
>> int i, num_algo;
>>
>> /* obtain algorithms supported by the TPM */
>> num_algo = tpm_get_pcr_banks_info(TPM_ANY_NUM, active_banks);
>>
>> for (i = 0; i < num_algo; i++) {
>> char *event = "application event";
>>
>> /* calculate event digest with current hash algorithm */
>> const char *algo_name = hash_algo_name[active_banks[i].crypto_id];
>> struct crypto_shash *tfm = crypto_alloc_shash(algo_name, 0, 0);
>>
>> app_calc_event_digest(tfm, event, digest_array[i].digest);
>> digest_array[i].alg_id = active_banks[i].alg_id;
>> }
>>
>> /* extend all PCR banks with calculated digests */
>> tpm_pcr_extend(TPM_ANY_NUM, 16, num_algo, digest_array);
>> }
>>
>>
>> Changelog
>>
>> v3
>>
>> - introduced new structure tpm_pcr_bank_info
>> - read a PCR to obtain the digest size for each TPM algorithm
>> - tpm_pcr_algorithms() replaced by tpm_get_pcr_banks_info()
>> - removed tpm_pcr_algo_to_crypto() and tpm_pcr_algo_from_crypto()
>> - modification of tpm_pcr_extend() parameters and of callers of the
>> function done in the same patch
>> - removed tpm_pcr_check_input()
>>
>> v2
>>
>> - tpm_pcr_algorithms() returns supported algorithms also for TPM 1.2
>> - fixed return values of tpm2_pcr_algo_to_crypto() and
>> tpm2_pcr_algo_from_crypto() if TPM support is disabled in the kernel
>> - tpm_pcr_extend() arguments checked by tpm_pcr_check_input()
>> - modified parameters of tpm_pcr_extend()
>>
>> Roberto Sassu (6):
>> tpm: use tpm_buf functions to perform a PCR read
>> tpm: use tpm2_pcr_read_tpm_buf() in tpm2_do_selftest()
>> tpm: introduce tpm_pcr_bank_info structure with digest_size from TPM
>> tpm: replace TPM algorithms IDs with tpm_pcr_bank_info structs in
>> tpm_chip
>> tpm: introduce tpm_get_pcr_banks_info()
>> tpm: pass multiple digests to tpm_pcr_extend()
>>
>> drivers/char/tpm/tpm-interface.c | 88 ++++++++++++++++++++++---
>> drivers/char/tpm/tpm.h | 19 +-----
>> drivers/char/tpm/tpm2-cmd.c | 132 ++++++++++++++++++++++---------------
>> include/linux/tpm.h | 39 ++++++++++-
>> security/integrity/ima/ima_queue.c | 4 +-
>> security/keys/trusted.c | 6 +-
>> 6 files changed, 202 insertions(+), 86 deletions(-)
>>
>> --
>> 2.9.3
>>
>> --
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>
>
> To move this forward and be more constructive here's how I see it
> should be done (along the lines, draft):
>
> int tpm_pcr_extend(u32 chip_num, int pcr_idx, unsigned int alg,
> const u8 *hash);
>
> The paramater 'alg' is crypto ID as specified by crypto subsystem.

You cannot pass one digest at time. Suppose that you want to extend
the SHA1 and SHA256 banks, each with a digest calculated with the
same algorithm. If you pass a SHA1 digest first, also the SHA256 bank
will be extended with the padded SHA1 digest.

Regarding crypto IDs, it has been decided that IMA will include
TPM algorithm IDs in the event log. So both IDs should be passed
to IMA somehow.

Roberto

> TPM driver must have a precompiled table of mappings for crypto IDs
> and TPM algorithm IDs.
>
> In addition it must have dynamically acquired list of TPM alg IDs.
> For those algs that static mapping does not exist it must extend
> them like we do now everything else except SHA-1 (Naynas changes).
>
> There's absolutely no need to pass digest size like you do BTW as it is
> defined by the standard.
>
> I also except that where ever this interleaves with trusted keys there
> won't be duplicate structures and code.
>
> /Jarkko
>

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