Re: [Linux-ima-devel] [PATCH 0/7] IMA: new parser for ima_restore_measurement_list()

From: Roberto Sassu
Date: Wed May 17 2017 - 03:25:34 EST

On 5/16/2017 9:00 PM, Ken Goldman wrote:
On 5/16/2017 8:53 AM, Roberto Sassu wrote:
A new IMA measurement list format, called Crypto Agile, will be introduced
shortly to take full advantage of the algorithm flexibility of TPM 2.0.
With the new format, it will be possible to provide for each list entry
multiple digests, each calculated with an algorithm supported by the TPM.
Those digests will be used by remote entities to verify the integrity of
the measurements list.

The current (SHA1) and the new (Crypto Agile) format definitions are:

SHA1: pcr[4] digest[20]
template_name_len[4] template_name[template_name_len]
template_data_len[4] template_data[template_data_len]

Crypto Agile: pcr[4] total_digest_len[4]
digest1_len[4] digest1[digest1_len] ...
digestN_len[4] digestN[digestN_len]
template_name_len[4] template_name[template_name_len]
template_data_len[4] template_data[template_data_len]

1 - In this proposed format, how does the parser or consumer of the log
know what algorithm is used for digestN.

The format of digestN is: <algo name>:\0<digest value>, the same used
for the file digest.

For example, the TCG standard format uses TPML_DIGEST_VALUES
uint32_t count - the number of digests TPMT_HA
TPMT_HA digests[]

where a TPMT_HA is
algorithm identifier
digest byte array

2 - Not a criticism, just a question for understanding ... Would it be
true that the total_digest_length == the sum of all the digestN_len
values plus 4 bytes for each length.


Does it determine how many digests there are by when the total length is

Yes. Since the number of digests is unknown until the buffer is parsed,
the parser consumes the data until the buffer end is reached. Then,
it returns the number of digests to the caller.