cryptenroll: explicitly pick PCR bank if literal PCR binding is off, but signed PCR binding is on

We so far derived the PCR bank to use from the PCR values specified fr
literal PCR binding. However, when that's not used then we left the bank
uninitialized – which will break if signed PCR binds are used (where we
need to pick a bank too after all).

Hence, let's explicitly pick a bank to use if literal PCR values are not
used, to make things just work.

Fixes: #32946

src/cryptenroll/cryptenroll-tpm2.c

@@ -371,8 +371,10 @@ int enroll_tpm2(struct crypt_device *cd,
 
         uint16_t hash_pcr_bank = 0;
         uint32_t hash_pcr_mask = 0;
+
         if (n_hash_pcr_values > 0) {
                 size_t hash_count;
+
                 r = tpm2_pcr_values_hash_count(hash_pcr_values, n_hash_pcr_values, &hash_count);
                 if (r < 0)
                         return log_error_errno(r, "Could not get hash count: %m");
@@ -380,10 +382,21 @@ int enroll_tpm2(struct crypt_device *cd,
                 if (hash_count > 1)
                         return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Multiple PCR banks selected.");
 
+                /* If we use a literal PCR value policy, derive the bank to use from the algorithm specified on the hash values */
                 hash_pcr_bank = hash_pcr_values[0].hash;
                 r = tpm2_pcr_values_to_mask(hash_pcr_values, n_hash_pcr_values, hash_pcr_bank, &hash_pcr_mask);
                 if (r < 0)
                         return log_error_errno(r, "Could not get hash mask: %m");
+        } else if (pubkey_pcr_mask != 0) {
+
+                /* If no literal PCR value policy is used, then let's determine the mask to use automatically
+                 * from the measurements of the TPM. */
+                r = tpm2_get_best_pcr_bank(
+                                tpm2_context,
+                                pubkey_pcr_mask,
+                                &hash_pcr_bank);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to determine best PCR bank: %m");
         }
 
         TPM2B_DIGEST policy = TPM2B_DIGEST_MAKE(NULL, TPM2_SHA256_DIGEST_SIZE);