Re: [PATCH v3 2/8] ima: Switch to ima_hash_algo for boot aggregate

[Mimi Zohar]

On Mon, 2020-02-10 at 11:00 +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). 

Up to here, the patch description matches the code.
> 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.

This comment and the one inline are left over from previous version.

> 
> Changelog
> 
> v1:
> - add Mimi's comments
> - if there is no PCR bank for the IMA default algorithm use SHA256
>   (suggested by James Bottomley)
> 
> Cc: stable@xxxxxxxxxxxxxxx # 5.1.x
> Fixes: 879b589210a9 ("tpm: retrieve digest size of unknown algorithms with PCR read")
> Reported-by: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>
> Suggested-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

Thanks, Roberto. ÂThis patch is dependent on 1/8. ÂAs soon as there's
a topic branch, I'll queue this, removing the extraneous comments.

Mimi

> ---
>  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);
> +
>  	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)
>  {
>  	struct crypto_shash *tfm;
> -	int rc;
> +	u16 crypto_id, alg_id;
> +	int rc, i, bank_idx = 0;
> +
> +	for (i = 0; i < ima_tpm_chip->nr_allocated_banks; i++) {
> +		crypto_id = ima_tpm_chip->allocated_banks[i].crypto_id;
> +		if (crypto_id == hash->algo) {
> +			bank_idx = i;
> +			break;
> +		}
> +
> +		if (crypto_id == HASH_ALGO_SHA256)
> +			bank_idx = i;
> +	}
> +
> +	if (bank_idx == 0 &&
> +	    ima_tpm_chip->allocated_banks[0].crypto_id == HASH_ALGO__LAST) {
> +		pr_err("No suitable TPM algorithm for boot aggregate\n");
> +		return 0;
> +	}
> +
> +	hash->algo = ima_tpm_chip->allocated_banks[bank_idx].crypto_id;
>  
>  	tfm = ima_alloc_tfm(hash->algo);
>  	if (IS_ERR(tfm))
>  		return PTR_ERR(tfm);
>  
>  	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);
>  
>  	ima_free_tfm(tfm);
>  
> diff --git a/security/integrity/ima/ima_init.c b/security/integrity/ima/ima_init.c
> index 5d55ade5f3b9..fbd7a8e28a6b 100644
> --- a/security/integrity/ima/ima_init.c
> +++ b/security/integrity/ima/ima_init.c
> @@ -27,7 +27,7 @@ struct tpm_chip *ima_tpm_chip;
>  /* Add the boot aggregate to the IMA measurement list and extend
>   * the PCR register.
>   *
> - * Calculate the boot aggregate, a SHA1 over tpm registers 0-7,
> + * Calculate the boot aggregate, a hash over tpm registers 0-7,
>   * assuming a TPM chip exists, and zeroes if the TPM chip does not
>   * exist.  Add the boot aggregate measurement to the measurement
>   * list and extend the PCR register.
> @@ -51,15 +51,27 @@ static int __init ima_add_boot_aggregate(void)
>  	int violation = 0;
>  	struct {
>  		struct ima_digest_data hdr;
> -		char digest[TPM_DIGEST_SIZE];
> +		char digest[TPM_MAX_DIGEST_SIZE];
>  	} hash;
>  
>  	memset(iint, 0, sizeof(*iint));
>  	memset(&hash, 0, sizeof(hash));
>  	iint->ima_hash = &hash.hdr;
> -	iint->ima_hash->algo = HASH_ALGO_SHA1;
> -	iint->ima_hash->length = SHA1_DIGEST_SIZE;
> -
> +	iint->ima_hash->algo = ima_hash_algo;
> +	iint->ima_hash->length = hash_digest_size[ima_hash_algo];
> +
> +	/*
> +	 * With TPM 2.0 hash agility, TPM chips could support multiple TPM
> +	 * PCR banks, allowing firmware to configure and enable different
> +	 * banks.  The SHA1 bank is not necessarily enabled.
> +	 *
> +	 * 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.  Otherwise, use the
> +	 * TPM required banks - SHA256 for TPM 2.0, SHA1 for TPM 1.2. If
> +	 * SHA256 is not available, use the first PCR bank and if that is
> +	 * not mapped to a crypto ID, set boot_aggregate to zero.

The last line of the above comment is left over from the previous
version.

> +	 */
>  	if (ima_tpm_chip) {
>  		result = ima_calc_boot_aggregate(&hash.hdr);
>  		if (result < 0) {