+ Roberto, David
Currently tpm_pcr_extend takes only 20 octets and "expands" if necessary
to fill the size of other pcr banks. We want to make IMA algorithm agile.
Current interface:
int tpm_pcr_extend(struct tpm_chip *chip, int pcr-idx, const u8 *hash)
{
...
}
Proposed:
//Or should we use the defined constants from other libraries, for now
// I'll use the TCG defined constants
#define TPM_DIGEST_SIZE_SHA1 20
#define TPM_DIGEST_SIZE_SHA256 32
// Will need to define TCG algorithm IDs in TPM driver-specific header.
// Alternatively we could use the IDs from IMA / Linux and make the driver
// do the mapping to TCG alg IDs
...
int tpm_pcr_extend(struct tpm_chip *chip, int pcr-idx, struct *pcr_bank_list)
{
// The driver will take the contents of the pcr_bank_list and create a
// TPML_DIGEST_VALUES structure to pass into the TPM2_PCR_Extend command
// See comment below about mismatched pcr banks
}
// The caller will allocate a static set of octet arrays, one for each allocated
// pcr bank
struct pcr_bank_list
{
uint16 tcg_hash_algid; // this is from the TCG algorthm registry
uint8 hash_size; // size of octect array
uint8 *extend_array; // pointer to array of octets to Extend.
struct list_head pcr_bank;
}
// Caller allocates the following for each PCR bank. Each is pointed to by
// parameter extend_array
unit8[TPM_DIGEST_SIZE_SHA1] hash_sha1;
unit8[TPM_DIGEST_SIZE_SHA256] hash_sha256;
....
Mismatched PCR banks:
There is no programatic method now to enforce a match between
the caller's set of pcr banks and the TPM's allocated banks. For example,
the caller might only be configured to produce SHA1 hashes but the
platform owner (via a firmware setting) may have allocated PCR banks
for SHA1 and SHA256
Therefore, we propose the following as a starting point in the dicussion
1> If a PCR bank is allocated but there is no matching entry in the list of
pcr_bank_list for the pcr allocated bank:
options:
A> Append 0's or truncate based on a set of pre-defined rules
(See below)
B> Extend the missing pcr bank with a constant
C> Don't extend the missing bank.
Option A:
Append 0's or truncate in the following order
TPM_ALG_SHA1 <-> TPM_ALG_SHA256 <-> TPM_ALG_SHA3_256 <-> TPM_ALG_SHA384 <->
TPM_ALG_SHA3_384 <-> TPM_ALG_SHA512 <-> TPM_ALG_SHA3_512
The question is what to do about TPM_ALG_SM3_256?
Option C:
This is a viable option and may actully be what the caller wants. There actually
is no rule the all banks must be extended. In fact when "sealing", the
caller lists
the pcr banks they want to seal to. (While is it technically possible
to provide the
TPM2_PolicyPCR a mix of banks I don't beieve this practical as only
one expected hash
is provided as input. We should consider this option.
Further, there is no TPM spec requirement to allocate banks for all
PCRs. For example the
following is allowed:
PCR Allocated 1 2 3 4
SHA1 bank allocated allocated allocated allocated
SHA256 bank allocated allocated Not allocated Not allocated
Monty
