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

[Mimi Zohar]

On Tue, 2020-02-04 at 13:37 +0000, Roberto Sassu wrote:
> > -----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).

Thanks! ÂAt this point we know the hash algorithm, so we could use
hash_digest_size[].

Mimi