On 12/5/2018 12:18 AM, Jarkko Sakkinen wrote:
On Tue, Dec 04, 2018 at 09:21:32AM +0100, Roberto Sassu wrote:
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.
...
+ 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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