Re: EXTERNAL: [PATCH v2 2/3] tpm: modify tpm_pcr_read() definition to pass TPM hash algorithms

[Mimi Zohar <zohar@linux.ibm.com>]

On Wed, 2018-09-05 at 17:03 +0200, Roberto Sassu wrote:
> On 9/5/2018 3:43 PM, Jeremy Boone wrote:
> > Some comments on tpm2_pcr_read below.
> > 
> > The tpm2_pcr_read function uses TPM2_ST_NO_SESSIONS. This means
> that the response payload is not integrity protected with an HMAC.
> If there is a man-in-the-middle sitting on the serial bus that
> connects the TPM peripheral to the processor, they can tamper with
> the response parameters.
> > 
> > In your changes to tpm2_pcr_read, the memcpy is now become a
> variable-length operation, instead of just copying a fixed number of
> bytes. If the MITM modifies the response field out->digest_size
> before it is received by the driver, they can make it a very large
> value, forcing a buffer overflow of the out->digest array.
> > 
> > Adding a session to the PCR Read command seems like overkill in
> this case. I wouldn't recommend that as a solution here.  So to fix
> this I would suggest simply checking the digest size before the
> memcpy.
> 
> Hi Jeremy
> 
> ok, thanks.

The hash digest size checking should be based on the size stored in
the active_bank_info, either in 3/3 or as a separate patch.

Mimi