tpm: fix handling of the TPM 2.0 event logs

When TPM2 log has entries with more than 3 digests, or with digests
not listed in the log header, log gets misparsed, eventually
leading to kernel complaint that code tried to vmalloc 512MB of
memory (I have no idea what would happen on bigger system).

So code should not parse only first 3 digests: both event header
and event itself are already in memory, so we can parse any number
of digests, as long as we do not try to parse whole memory when
given count of 0xFFFFFFFF.

So this change:

* Rejects event entry with more digests than log header describes.
  Digest types should be unique, and all should be described in
  log header, so there cannot be more digests in the event than in
  the header.

* Reject event entry with digest that is not described in the
  log header.  In theory code could hardcode information about
  digest IDs already assigned by TCG, but if firmware authors
  cannot get event log format right, why should anyone believe
  that they got event log content right.

Cc: stable@vger.kernel.org
Fixes: 4d23cc32 ("tpm: add securityfs support for TPM 2.0 firmware event log")
Signed-off-by: Petr Vandrovec <petr@vmware.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

drivers/char/tpm/tpm2_eventlog.c

@@ -56,18 +56,24 @@ static int calc_tpm2_event_size(struct tcg_pcr_event2 *event,
 
 	efispecid = (struct tcg_efi_specid_event *)event_header->event;
 
-	for (i = 0; (i < event->count) && (i < TPM2_ACTIVE_PCR_BANKS);
-	     i++) {
+	/* Check if event is malformed. */
+	if (event->count > efispecid->num_algs)
+		return 0;
+
+	for (i = 0; i < event->count; i++) {
 		halg_size = sizeof(event->digests[i].alg_id);
 		memcpy(&halg, marker, halg_size);
 		marker = marker + halg_size;
-		for (j = 0; (j < efispecid->num_algs); j++) {
+		for (j = 0; j < efispecid->num_algs; j++) {
 			if (halg == efispecid->digest_sizes[j].alg_id) {
-				marker = marker +
+				marker +=
 					efispecid->digest_sizes[j].digest_size;
 				break;
 			}
 		}
+		/* Algorithm without known length. Such event is unparseable. */
+		if (j == efispecid->num_algs)
+			return 0;
 	}
 
 	event_field = (struct tcg_event_field *)marker;