Re: [Linux-ima-devel] [PATCH v2 3/5] tpm: pass multiple digests to tpm_pcr_extend()

[Roberto Sassu]

On 5/30/2017 1:25 PM, Mimi Zohar wrote:
> On Tue, 2017-05-30 at 09:28 +0200, Roberto Sassu wrote:
> > On 5/30/2017 5:29 AM, Mimi Zohar wrote:
> > > On Fri, 2017-05-05 at 16:21 +0200, Roberto Sassu wrote:
>
>
> > > > @@ -876,29 +925,46 @@ static int tpm1_pcr_extend(struct tpm_chip *chip, int pcr_idx, const u8 *hash,
> > > >   * isn't, protect against the chip disappearing, by incrementing
> > > >   * the module usage count.
> > > >   */
> > > > -int tpm_pcr_extend(u32 chip_num, int pcr_idx, const u8 *hash)
> > > > +int tpm_pcr_extend(u32 chip_num, int pcr_idx, int count,
> > > > +		   struct tpm2_digest *digests)
> > > >  {
> > > >  	int rc;
> > > >  	struct tpm_chip *chip;
> > > >  	struct tpm2_digest digest_list[ARRAY_SIZE(chip->active_banks)];
> > > > -	u32 count = 0;
> > > > +	struct tpm2_digest *digests_ptr = digests;
> > > > +	u32 filled_count = 0;
> > > > +	u8 *hash;
> > > >  	int i;
> > > >
> > > >  	chip = tpm_chip_find_get(chip_num);
> > > >  	if (chip == NULL)
> > > >  		return -ENODEV;
> > > >
> > > > -	if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> > > > +	rc = tpm_pcr_check_input(chip, count, digests);
> > > > +	if (rc < 0) {
> > > > +		dev_dbg(&chip->dev, "%s: invalid arguments\n", __func__);
> > > > +		tpm_put_ops(chip);
> > >
> > > This rejects the TPM extend, if ANY of the algorithms are unknown.
> > > Suppose that the standards were updated, TPM vendors add support for
> > > the new algorithm, but the kernel has not been updated to reflect the
> > > new algorithms supported.  As the measurement hash already been added
> > > to the IMA measurement list, verifying the measurement list against a
> > > TPM quote will fail, not just for the unknown algorithm, but for all
> > > algorithms.  Something is very broken with this approach.
> >
> > The alternative is to extend remaining banks with a digest,
> > for example the first passed by the caller. I will modify
> > the patch, if everyone agrees on that.
>
> The solution you're proposing is similar to the original solution of
> extending the TPM with a padded/truncated SHA1 hash, but this time it
> might not be a padded/truncated SHA1 hash, but a different algorithm.
> So the attestation server will then need to know which hash algorithm
> was used to extend each of the TPM banks - a padded/truncated digest
> value or the real digest value.

The convention would be that the first digest in the event log
is truncated/padded, to extend banks for which no digest is
provided. This convention would apply to all callers of
tpm_pcr_extend().

Roberto


> The only issue, from the kernel's perspective, will be determining the
> algorithm's digest size as the kernel has no knowledge of it.  On TPM
> registration/initialization, if you're not already querying the TPM
> for the algorithm digest sizes, you will need to do so.
>
> Mimi

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