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

Roberto Sassu
2017-05-30T13:25:47Z

On 5/30/2017 1:25 PM, Mimi Zohar wrote:
> On Tue, 2017-05-30 at 09:28 +0200, Roberto Sassu wrote:
>> On 5/30/2017 5:29 AM, Mimi Zohar wrote:
>>> On Fri, 2017-05-05 at 16:21 +0200, Roberto Sassu wrote:
>
>
>>>> @@ -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.
>
> The solution you're proposing is similar to the original solution of
> extending the TPM with a padded/truncated SHA1 hash, but this time it
> might not be a padded/truncated SHA1 hash, but a different algorithm.
> So the attestation server will then need to know which hash algorithm
> was used to extend each of the TPM banks - a padded/truncated digest
> value or the real digest value.

The convention would be that the first digest in the event log
is truncated/padded, to extend banks for which no digest is
provided. This convention would apply to all callers of
tpm_pcr_extend().

Roberto

> The only issue, from the kernel's perspective, will be determining the
> algorithm's digest size as the kernel has no knowledge of it. On TPM
> registration/initialization, if you're not already querying the TPM
> for the algorithm digest sizes, you will need to do so.
>
> Mimi
>

--
HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Bo PENG, Qiuen PENG, Shengli WANG