Re: [PATCH v2 2/8] ima: Switch to ima_hash_algo for boot aggregate

[Mimi Zohar]

On Wed, 2020-02-05 at 11:33 +0100, Roberto Sassu wrote:
> boot_aggregate is the first entry of IMA measurement list. Its purpose is
> to link pre-boot measurements to IMA measurements. As IMA was designed to
> work with a TPM 1.2, the SHA1 PCR bank was always selected.
> 
> Currently, even if a TPM 2.0 is used, the SHA1 PCR bank is selected.
> However, the assumption that the SHA1 PCR bank is always available is not
> correct, as PCR banks can be selected with the PCR_Allocate() TPM command.
> 
> This patch tries to use ima_hash_algo as hash algorithm for boot_aggregate.
> If no PCR bank uses that algorithm, the patch tries to find the SHA256 PCR
> bank (which is mandatory in the TCG PC Client specification). If also this
> bank is not found, the patch selects the first one. If the TPM algorithm
> of that bank is not mapped to a crypto ID, boot_aggregate is set to zero.
> 
> Changelog
> 
> v1:
> - add Mimi's comments
> - if there is no PCR bank for the IMA default algorithm use SHA256 or the
>   first bank (suggested by James Bottomley)

If the IMA default hash algorithm is not enabled, James' comment was
to use SHA256 for TPM 2.0 and SHA1 for TPM 1.2. ÂI don't remember him
saying anything about using the first bank, that was in v1 of my
patch. ÂPlease refer to v2 of my patch, based on James' comments.

> 
> Reported-by: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>
> Suggested-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  security/integrity/ima/ima_crypto.c | 45 +++++++++++++++++++++++++----
>  security/integrity/ima/ima_init.c   | 22 ++++++++++----
>  2 files changed, 56 insertions(+), 11 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
> index 73044fc6a952..f2f41a2bc3d4 100644
> --- a/security/integrity/ima/ima_crypto.c
> +++ b/security/integrity/ima/ima_crypto.c
> @@ -655,18 +655,29 @@ static void __init ima_pcrread(u32 idx, struct tpm_digest *d)
>  }
>  
>  /*
> - * Calculate the boot aggregate hash
> + * The boot_aggregate is a cumulative hash over TPM registers 0 - 7.  With
> + * TPM 1.2 the boot_aggregate was based on reading the SHA1 PCRs, but with
> + * TPM 2.0 hash agility, TPM chips could support multiple TPM PCR banks,
> + * allowing firmware to configure and enable different banks.
> + *
> + * Knowing which TPM bank is read to calculate the boot_aggregate digest
> + * needs to be conveyed to a verifier.  For this reason, use the same
> + * hash algorithm for reading the TPM PCRs as for calculating the boot
> + * aggregate digest as stored in the measurement list.
>   */
> -static int __init ima_calc_boot_aggregate_tfm(char *digest,
> +static int __init ima_calc_boot_aggregate_tfm(char *digest, u16 alg_id,
>  					      struct crypto_shash *tfm)
>  {
> -	struct tpm_digest d = { .alg_id = TPM_ALG_SHA1, .digest = {0} };
> +	struct tpm_digest d = { .alg_id = alg_id, .digest = {0} };
>  	int rc;
>  	u32 i;
>  	SHASH_DESC_ON_STACK(shash, tfm);
>  
>  	shash->tfm = tfm;
>  
> +	pr_devel("calculating the boot-aggregate based on TPM bank: %04x\n",
> +		 d.alg_id);
> +

Good

>  	rc = crypto_shash_init(shash);
>  	if (rc != 0)
>  		return rc;
> @@ -675,7 +686,8 @@ static int __init ima_calc_boot_aggregate_tfm(char *digest,
>  	for (i = TPM_PCR0; i < TPM_PCR8; i++) {
>  		ima_pcrread(i, &d);
>  		/* now accumulate with current aggregate */
> -		rc = crypto_shash_update(shash, d.digest, TPM_DIGEST_SIZE);
> +		rc = crypto_shash_update(shash, d.digest,
> +					 crypto_shash_digestsize(tfm));
>  	}
>  	if (!rc)
>  		crypto_shash_final(shash, digest);
> @@ -685,14 +697,35 @@ static int __init ima_calc_boot_aggregate_tfm(char *digest,
>  int __init ima_calc_boot_aggregate(struct ima_digest_data *hash)
>  {
> 
< snip >
>  
>  	hash->length = crypto_shash_digestsize(tfm);
> -	rc = ima_calc_boot_aggregate_tfm(hash->digest, tfm);
> +	alg_id = ima_tpm_chip->allocated_banks[bank_idx].alg_id;
> +	rc = ima_calc_boot_aggregate_tfm(hash->digest, alg_id, tfm);

Sure, backporting this change to ima_calc_boot_aggregate_tfm() should
be fine.

Mimi

>  	ima_free_tfm(tfm);
>