RE: [PATCH v3 2/2] ima: support calculating the boot_aggregate based on different TPM banks

[Roberto Sassu]

> -----Original Message-----
> From: linux-integrity-owner@xxxxxxxxxxxxxxx [mailto:linux-integrity-
> owner@xxxxxxxxxxxxxxx] On Behalf Of Mimi Zohar
> Sent: Thursday, January 30, 2020 5:23 PM
> To: linux-integrity@xxxxxxxxxxxxxxx
> Cc: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>; James Bottomley
> <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx; Mimi Zohar <zohar@xxxxxxxxxxxxx>
> Subject: [PATCH v3 2/2] ima: support calculating the boot_aggregate based
> on different TPM banks
> 
> Calculating the boot_aggregate attempts to read the TPM SHA1 bank,
> assuming it is always enabled.  With TPM 2.0 hash agility, TPM chips
> could support multiple TPM PCR banks, allowing firmware to configure and
> enable different banks.
> 
> Instead of hard coding the TPM 2.0 bank hash algorithm used for calculating
> the boot-aggregate, use the same hash algorithm for reading the TPM PCRs
> as
> for calculating the boot aggregate digest.  Preference is given to the
> configured IMA default hash algorithm.
> 
> For TPM 1.2 SHA1 is the only supported hash algorithm.
> 
> Reported-by: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>
> Suggested-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> ---
>  security/integrity/ima/ima_crypto.c | 45
> ++++++++++++++++++++++++++++++++++++-
>  1 file changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/security/integrity/ima/ima_crypto.c
> b/security/integrity/ima/ima_crypto.c
> index 7967a6904851..a020aaefdea8 100644
> --- a/security/integrity/ima/ima_crypto.c
> +++ b/security/integrity/ima/ima_crypto.c
> @@ -656,13 +656,34 @@ static void __init ima_pcrread(u32 idx, struct
> tpm_digest *d)
>  		pr_err("Error Communicating to TPM chip\n");
>  }
> 
> +static enum hash_algo get_hash_algo(const char *algname)
> +{
> +	int i;
> +
> +	for (i = 0; i < HASH_ALGO__LAST; i++) {
> +		if (strcmp(algname, hash_algo_name[i]) == 0)
> +			break;
> +	}
> +
> +	return i;
> +}
> +
>  /*
> - * 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,
>  					      struct crypto_shash *tfm)
>  {
>  	struct tpm_digest d = { .alg_id = TPM_ALG_SHA1, .digest = {0} };
> +	enum hash_algo crypto_id;
>  	int rc;
>  	u32 i;
>  	SHASH_DESC_ON_STACK(shash, tfm);
> @@ -673,6 +694,28 @@ static int __init ima_calc_boot_aggregate_tfm(char
> *digest,
>  	if (rc != 0)
>  		return rc;
> 
> +	crypto_id = get_hash_algo(crypto_shash_alg_name(tfm));
> +	if (crypto_id == HASH_ALGO__LAST) {
> +		pr_devel("unknown hash algorithm (%s), failing to read
> PCRs.\n",
> +			 crypto_shash_alg_name(tfm));
> +		return 0;
> +	}
> +
> +	for (i = 0; i < ima_tpm_chip->nr_allocated_banks; i++) {
> +		if (ima_tpm_chip->allocated_banks[i].crypto_id ==
> crypto_id) {
> +			d.alg_id = ima_tpm_chip->allocated_banks[i].alg_id;
> +			break;
> +		}
> +	}
> +	if (i == ima_tpm_chip->nr_allocated_banks) {
> +		pr_devel("TPM %s bank not allocated, failing to read
> PCRs.\n",
> +			 crypto_shash_alg_name(tfm));
> +		return 0;
> +	}
> +
> +	pr_devel("calculating the boot-aggregregate based on TPM
> bank: %04x\n",
> +		  d.alg_id);
> +
>  	/* cumulative sha1 over tpm registers 0-7 */
>  	for (i = TPM_PCR0; i < TPM_PCR8; i++) {
>  		ima_pcrread(i, &d);

The third argument of crypto_shash_update() should be
crypto_shash_digestsize(tfm).

Roberto

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