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

[Mimi Zohar]

On Tue, 2020-01-28 at 14:19 +0000, Roberto Sassu wrote:
> > -----Original Message-----
> > From: linux-integrity-owner@xxxxxxxxxxxxxxx [mailto:linux-integrity-
> > owner@xxxxxxxxxxxxxxx] On Behalf Of Mimi Zohar
> > Sent: Monday, January 27, 2020 5:02 PM
> > To: linux-integrity@xxxxxxxxxxxxxxx
> > Cc: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>; James Bottomley
> > <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>; linux-
> > kernel@xxxxxxxxxxxxxxx; Mimi Zohar <zohar@xxxxxxxxxxxxx>
> > Subject: [PATCH 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, see if the configured IMA_DEFAULT_HASH algorithm is
> > an allocated TPM bank, otherwise use the first allocated TPM bank.
> > 
> > For TPM 1.2 SHA1 is the only supported hash algorithm.
> > 
> > Reported-by: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>
> > Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> > ---
> >  security/integrity/ima/ima_crypto.c | 37
> > ++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 36 insertions(+), 1 deletion(-)
> > 
> > diff --git a/security/integrity/ima/ima_crypto.c
> > b/security/integrity/ima/ima_crypto.c
> > index 7967a6904851..b1b26d61f174 100644
> > --- a/security/integrity/ima/ima_crypto.c
> > +++ b/security/integrity/ima/ima_crypto.c
> > @@ -656,8 +656,25 @@ static void __init ima_pcrread(u32 idx, struct
> > tpm_digest *d)
> >  		pr_err("Error Communicating to TPM chip\n");
> >  }
> > 
> > +/* tpm2_hash_map is the same as defined in tpm2-cmd.c and
> > trusted_tpm2.c */
> > +static struct tpm2_hash tpm2_hash_map[] = {
> > +	{HASH_ALGO_SHA1, TPM_ALG_SHA1},
> > +	{HASH_ALGO_SHA256, TPM_ALG_SHA256},
> > +	{HASH_ALGO_SHA384, TPM_ALG_SHA384},
> > +	{HASH_ALGO_SHA512, TPM_ALG_SHA512},
> > +	{HASH_ALGO_SM3_256, TPM_ALG_SM3_256},
> > +};
> > +
> >  /*
> > - * Calculate the boot aggregate hash
> > + * The boot_aggregate is a cumulative hash over TPM registers 0 - 7.  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 bank hash algorithm used for calculating
> > + * the boot-aggregate, see if the configured IMA_DEFAULT_HASH
> > algorithm is
> > + * an allocated TPM bank, otherwise use the first allocated TPM bank.
> > + *
> > + * For TPM 1.2 SHA1 is the only hash algorithm.
> >   */
> >  static int __init ima_calc_boot_aggregate_tfm(char *digest,
> >  					      struct crypto_shash *tfm)
> > @@ -673,6 +690,24 @@ static int __init ima_calc_boot_aggregate_tfm(char
> > *digest,
> >  	if (rc != 0)
> >  		return rc;
> > 
> > +	for (i = 0; i < ARRAY_SIZE(tpm2_hash_map); i++) {
> > +		if (tpm2_hash_map[i].crypto_id == ima_hash_algo) {
> 
> It is not necessary to define a new map. ima_tpm_chip->allocated_banks
> has a crypto_id field.

Ok, thanks.

> 
> > +			d.alg_id = tpm2_hash_map[i].tpm_id;
> > +			break;
> > +		}
> > +	}
> > +
> > +	for (i = 0; i < ima_tpm_chip->nr_allocated_banks; i++) {
> > +		if (ima_tpm_chip->allocated_banks[i].alg_id == d.alg_id)
> > +			break;
> > +	}
> > +
> > +	if (i == ima_tpm_chip->nr_allocated_banks)
> > +		d.alg_id = ima_tpm_chip->allocated_banks[0].alg_id;
> 
> This code assumes that the algorithm used to calculate boot_aggregate and
> the algorithm of the PCR bank can be different. I don't know if it is possible to
> communicate to the verifier which bank has been selected (it depends on
> the local configuration).

Agreed, but defaulting to the first bank would only happen if the IMA
default hash algorithm is not a configured TPM algorithm.

> 
> In my opinion the safest approach would be to use the same algorithm for the
> digest and the PCR bank. If you agree to this, then the code above must be
> moved to ima_calc_boot_aggregate() so that the algorithm of the selected
> PCR bank can be passed to ima_alloc_tfm().

Using the same hash algorithm, preferably the IMA hash default
algorithm, for reading the TPM PCR bank and calculating the
boot_aggregate makes sense.

> 
> The selected PCR bank might be not the first, if the algorithm is unknown to
> the crypto subsystem.

It sounds like you're suggesting finding a common configured hash
algorithm between the TPM and the kernel.Â

> 
> > +	pr_info("Calculating the boot-aggregregate, reading TPM PCR
> 
> Typo.

thanks

Mimi