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

[Roberto Sassu]

> -----Original Message-----
> From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> Sent: Monday, February 10, 2020 11:24 PM
> To: Roberto Sassu <roberto.sassu@xxxxxxxxxx>;
> James.Bottomley@xxxxxxxxxxxxxxxxxxxxx;
> jarkko.sakkinen@xxxxxxxxxxxxxxx
> Cc: linux-integrity@xxxxxxxxxxxxxxx; linux-security-module@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; Silviu Vlasceanu
> <Silviu.Vlasceanu@xxxxxxxxxx>; stable@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v3 2/8] ima: Switch to ima_hash_algo for boot
> aggregate
> 
> 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.

Hi Mimi

actually the code does what is described above. bank_idx is initially
set to zero and remains as it is if there is no PCR bank for the default
IMA algorithm or SHA256.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> > 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) {