Re: [PATCH 0/8] ima: support stronger algorithms for attestation

Mimi Zohar
2020-01-30T22:26:57Z

Hi Roberto,

On Mon, 2020-01-27 at 18:04 +0100, Roberto Sassu wrote:
> IMA extends Platform Configuration Registers (PCRs) of the TPM to give a
> proof to a remote verifier that the measurement list contains all
> measurements done by the kernel and that the list was not maliciously
> modified by an attacker.
>
> IMA was originally designed to extend PCRs with a SHA1 digest, provided
> with the measurement list, and was subsequently updated to extend all PCR
> banks in case a TPM 2.0 is used. Non-SHA1 PCR banks are not supposed to be
> used for remote attestation, as they are extended with a SHA1 digest padded
> with zeros, which does not increase the strength.
>
> This patch set addresses this issue by extending PCRs with the digest of
> the measurement entry calculated with the crypto subsystem. The list of
> algorithms used to calculate the digest are taken from
> ima_tpm_chip->allocated_banks, returned by the TPM driver. The SHA1 digest
> is always calculated, as SHA1 still remains the default algorithm for the
> template digest in the measurement list.
>
> This patch set also makes two additional modifications related to the usage
> of hash algorithms. First, since now the template digest for the default
> IMA algorithm is always calculated, this is used for hash collision
> detection, to check if there are duplicate measurement entries.
>
> Second, it uses the default IMA hash algorithm to calculate the boot
> aggregate, assuming that the corresponding PCR bank is currently allocated.
> Otherwise, it finds the first PCR bank for which the crypto ID is known.
> IMA initialization fails only if no algorithm known to the crypto subsystem
> is found.
>
> This patch set does not yet modify the format of the measurement list to
> provide the digests passed to the TPM. However, reconstructing the value of
> the quoted PCR is still possible for the verifier by calculating the digest
> on measurement data found in binary_runtime_measurements.

Thank you!  I'm still reviewing and testing the patches, but it is
really nicely written.

Mimi