RE: [PATCH v3 7/8] ima: Calculate and extend PCR with digests in ima_template_entry

[Roberto Sassu <roberto.sassu@huawei.com>]

> -----Original Message-----
> From: linux-integrity-owner@vger.kernel.org [mailto:linux-integrity-
> owner@vger.kernel.org] On Behalf Of Mimi Zohar
> Sent: Wednesday, March 18, 2020 10:55 PM
> To: Roberto Sassu <roberto.sassu@huawei.com>;
> James.Bottomley@HansenPartnership.com;
> jarkko.sakkinen@linux.intel.com
> Cc: linux-integrity@vger.kernel.org; linux-security-module@vger.kernel.org;
> linux-kernel@vger.kernel.org; Silviu Vlasceanu
> <Silviu.Vlasceanu@huawei.com>
> Subject: Re: [PATCH v3 7/8] ima: Calculate and extend PCR with digests in
> ima_template_entry
> 
> On Wed, 2020-03-18 at 12:42 +0000, Roberto Sassu wrote:
> > > -----Original Message-----
> > > From: owner-linux-security-module@vger.kernel.org [mailto:owner-
> linux-
> > > security-module@vger.kernel.org] On Behalf Of Mimi Zohar
> > > Sent: Tuesday, March 3, 2020 5:04 AM
> > > To: Roberto Sassu <roberto.sassu@huawei.com>;
> > > James.Bottomley@HansenPartnership.com;
> > > jarkko.sakkinen@linux.intel.com
> > > Cc: linux-integrity@vger.kernel.org; linux-security-
> module@vger.kernel.org;
> > > linux-kernel@vger.kernel.org; Silviu Vlasceanu
> > > <Silviu.Vlasceanu@huawei.com>
> > > Subject: Re: [PATCH v3 7/8] ima: Calculate and extend PCR with digests
> in
> > > ima_template_entry
> > >
> > > On Mon, 2020-02-10 at 11:04 +0100, Roberto Sassu wrote:
> > >
> > > > @@ -219,6 +214,8 @@ int ima_restore_measurement_entry(struct
> > > ima_template_entry *entry)
> > > >
> > > >  int __init ima_init_digests(void)
> > > >  {
> > > > +	u16 digest_size;
> > > > +	u16 crypto_id;
> > > >  	int i;
> > > >
> > > >  	if (!ima_tpm_chip)
> > > > @@ -229,8 +226,17 @@ int __init ima_init_digests(void)
> > > >  	if (!digests)
> > > >  		return -ENOMEM;
> > > >
> > > > -	for (i = 0; i < ima_tpm_chip->nr_allocated_banks; i++)
> > > > +	for (i = 0; i < ima_tpm_chip->nr_allocated_banks; i++) {
> > > >  		digests[i].alg_id = ima_tpm_chip->allocated_banks[i].alg_id;
> > > > +		digest_size = ima_tpm_chip->allocated_banks[i].digest_size;
> > > > +		crypto_id = ima_tpm_chip->allocated_banks[i].crypto_id;
> > > > +
> > > > +		/* for unmapped TPM algorithms digest is still a padded
> > > SHA1 */
> > > > +		if (crypto_id == HASH_ALGO__LAST)
> > > > +			digest_size = SHA1_DIGEST_SIZE;
> > > > +
> > > > +		memset(digests[i].digest, 0xff, digest_size);
> > >
> > > Shouldn't the memset here be of the actual digest size even for
> > > unmapped TPM algorithms.
> >
> > This is consistent with ima_calc_field_array_hash(), so that a verifier
> > will always pad the SHA1 digest with zeros to obtain the final PCR value.
> >
> > I can set all bytes if you prefer.
> 
> My concern is with violations.  The measurement list will be padded
> with 0's, but the value being extended into the TPM will only
> partially be 0xFF's.  When verifying the measurement list, replacing
> all 0x00's with all 0xFF's is simpler.

If the TPM algorithm is unknown, the starting point is the SHA1 digest.
If there is a violation, this should be the one to be modified. Then, after
that, padding is done for all entries in the same way, regardless of
whether the entry is a violation or not.

Roberto

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