Re: [PATCH v4 6/6] tpm: ensure that the output of PCR read contains the correct digest size

[Jarkko Sakkinen]

On Tue, Nov 13, 2018 at 02:08:39PM +0100, Roberto Sassu wrote:
> On 11/8/2018 3:08 PM, Jarkko Sakkinen wrote:
> > On Tue, Nov 06, 2018 at 04:01:59PM +0100, Roberto Sassu wrote:
> > > This patch protects against data corruption that could happen in the bus,
> > > by checking that that the digest size returned by the TPM during a PCR read
> > > matches the size of the algorithm passed as argument to tpm2_pcr_read().
> > > 
> > > This check is performed after information about the PCR banks has been
> > > retrieved.
> > > 
> > > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > > ---
> > >   drivers/char/tpm/tpm2-cmd.c | 16 +++++++++++++++-
> > >   1 file changed, 15 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> > > index e2d5b84286a7..3b0b5b032901 100644
> > > --- a/drivers/char/tpm/tpm2-cmd.c
> > > +++ b/drivers/char/tpm/tpm2-cmd.c
> > > @@ -187,15 +187,28 @@ struct tpm2_pcr_read_out {
> > >   int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx,
> > >   		  struct tpm_digest *digest_struct, u16 *digest_size_ptr)
> > >   {
> > > +	int i;
> > >   	int rc;
> > >   	struct tpm_buf buf;
> > >   	struct tpm2_pcr_read_out *out;
> > >   	u8 pcr_select[TPM2_PCR_SELECT_MIN] = {0};
> > >   	u16 digest_size;
> > > +	u16 expected_digest_size = 0;
> > >   	if (pcr_idx >= TPM2_PLATFORM_PCR)
> > >   		return -EINVAL;
> > > +	if (!digest_size_ptr) {
> > > +		for (i = 0; i < chip->nr_active_banks &&
> > > +		     chip->active_banks[i].alg_id != digest_struct->alg_id; i++)
> > > +			;
> > 
> > Not sure if the semicolon should be in its own line.
> > `
> > > +
> > > +		if (i == chip->nr_active_banks)
> > > +			return -EINVAL;
> > > +
> > > +		expected_digest_size = chip->active_banks[i].digest_size;
> > > +	}
> > > +
> > >   	rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_PCR_READ);
> > >   	if (rc)
> > >   		return rc;
> > > @@ -215,7 +228,8 @@ int tpm2_pcr_read(struct tpm_chip *chip, int pcr_idx,
> > >   	out = (struct tpm2_pcr_read_out *)&buf.data[TPM_HEADER_SIZE];
> > >   	digest_size = be16_to_cpu(out->digest_size);
> > > -	if (digest_size > sizeof(digest_struct->digest)) {
> > > +	if (digest_size > sizeof(digest_struct->digest) ||
> > > +	    (!digest_size_ptr && digest_size != expected_digest_size)) {
> > >   		rc = -EINVAL;
> > >   		goto out;
> > >   	}
> > > -- 
> > > 2.17.1
> > > 
> > 
> > Please add
> > 
> > Cc: stable@xxxxxxxxxxxxxxxx
> 
> Should I do the same for the previous patches? This patch cannot be
> applied alone.
> 
> Roberto

No need. It is an issue that we deal with depenendent commits once it is
being backported. This could be dependent for example of a commit that
is even not in the series so does not make sense to do it now.

/Jarkko