Re: [PATCH v6 1/7] tpm: dynamically allocate the allocated_banks array

On 12/5/2018 12:18 AM, Jarkko Sakkinen wrote:

On Tue, Dec 04, 2018 at 09:21:32AM +0100, Roberto Sassu wrote:

quoted

tpm2_get_pcr_allocation() determines if a PCR bank is allocated by checking
the mask in the TPML_PCR_SELECTION structure returned by the TPM for
TPM2_Get_Capability(). One PCR bank with algorithm set to SHA1 is always
allocated for TPM 1.x.

...

quoted

+		for (j = 0; j < pcr_selection.size_of_select; j++)
+			if (pcr_selection.pcr_select[j])
+				break;
+
+		if (j < pcr_selection.size_of_select) {
+			chip->allocated_banks[nr_alloc_banks] = hash_alg;
+			nr_alloc_banks++;
+		}
+

Why was this needed? Can CAP_PCRS return completely unallocated banks?

This was discussed for patch v4 1/6:
---

Nayna wrote:

# /usr/local/bin/tssgetcapability -cap 5
2 PCR selections
    hash TPM_ALG_SHA1
    TPMS_PCR_SELECTION length 3
    ff ff ff
    hash TPM_ALG_SHA256
    TPMS_PCR_SELECTION length 3
    00 00 00

The pcr_select fields - "ff ff ff" and "00 00 00" - are bit masks for
the enabled PCRs. The SHA1 bank is enabled for all PCRs (0-23), while
the SHA256 bank is not enabled.
---

Kind of out-of-context for the rest of the changes.

Should this be a bug fix of its own because it looks like as this is a
bug fix for existing code, and not a new feature? Just asking because
I don't yet fully understand this change.

If we store in tpm_chip the possible banks, IMA would calculate more
digests unnecessarily. But this problem does not happen without my patch
set, because tpm_pcr_extend() only accepts a SHA1 digest.

Anyway, I believe that you can streamline this by:

/* Check that at least some of the PCRs have been allocated. This is
  * required because CAP_PCRS ...
  */
if (memchr_inv(pcr_selection.pcr_select, 0, pcr_selection.size_of_select))
	nr_allocated_banks++;

[yeah, comment would be awesome about CAP_PCRS. Did not finish up the
comment because I don't know the answer]

In addition, it would be consistent to call the local variable also
nr_allocated_banks (not nr_alloc_banks).

Unfortunately, I exceed the limit of characters per line.

Roberto

/Jarkko

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