Don't exercise if the algorithm is not supported

Parsing a key and importing it into PSA may result in a policy that
specifies an algorithm that is not included in the build. This happens if
the key type is supported, but not the algorithm, e.g. in a build with
MBEDTLS_ECP_C but not MBEDTLS_ECDSA_C.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/include/test/psa_exercise_key.h

@@ -221,4 +221,20 @@ int mbedtls_test_psa_exercise_key(mbedtls_svc_key_id_t key,
 psa_key_usage_t mbedtls_test_psa_usage_to_exercise(psa_key_type_t type,
                                                    psa_algorithm_t alg);
 
+/** Whether the specified algorithm can be exercised.
+ *
+ * \note This function is solely based on the algorithm and does not
+ *       consider potential issues with the compatibility of a key.
+ *       The idea is that you already have a key, so you know that the
+ *       key type is supported, and you want to exercise the key but
+ *       only if the algorithm given in its policy is enabled in the
+ *       compile-time configuration.
+ *
+ * \note This function currently only supports signature algorithms
+ *       (including wildcards).
+ *       TODO: a more general mechanism, which should be automatically
+ *       generated and possibly available as a library function?
+ */
+int mbedtls_test_can_exercise_psa_algorithm(psa_algorithm_t alg);
+
 #endif /* PSA_EXERCISE_KEY_H */

tests/src/psa_exercise_key.c

@@ -1009,4 +1009,49 @@ psa_key_usage_t mbedtls_test_psa_usage_to_exercise(psa_key_type_t type,
 
 }
 
+int mbedtls_test_can_exercise_psa_algorithm(psa_algorithm_t alg)
+{
+    /* Reject algorithms that we know are not supported. Default to
+     * attempting exercise, so that if an algorithm is missing from this
+     * function, the result will be a test failure and not silently
+     * omitting exercise. */
+#if !defined(PSA_WANT_ALG_RSA_PKCS1V15_CRYPT)
+    if (alg == PSA_ALG_RSA_PKCS1V15_CRYPT) {
+        return 0;
+    }
+#endif
+#if !defined(PSA_WANT_ALG_RSA_PKCS1V15_SIGN)
+    if (PSA_ALG_IS_RSA_PKCS1V15_SIGN(alg)) {
+        return 0;
+    }
+#endif
+#if !defined(PSA_WANT_ALG_RSA_PSS)
+    if (PSA_ALG_IS_RSA_PSS_STANDARD_SALT(alg)) {
+        return 0;
+    }
+#endif
+#if !defined(PSA_WANT_ALG_RSA_PSS_ANY_SALT)
+    if (PSA_ALG_IS_RSA_PSS_ANY_SALT(alg)) {
+        return 0;
+    }
+#endif
+#if !defined(PSA_WANT_ALG_ECDSA)
+    if (PSA_ALG_IS_ECDSA(alg)) {
+        return 0;
+    }
+#endif
+#if !defined(PSA_WANT_ALG_DETERMINISTIC_ECDSA)
+    if (PSA_ALG_IS_DETERMINISTIC_ECDSA(alg)) {
+        return 0;
+    }
+#endif
+#if !defined(PSA_WANT_ALG_ECDH)
+    if (PSA_ALG_IS_ECDH(alg)) {
+        return 0;
+    }
+#endif
+    (void) alg;
+    return 1;
+}
+
 #endif /* MBEDTLS_PSA_CRYPTO_C */

tests/suites/test_suite_pkparse.function

@@ -48,10 +48,14 @@ static int test_psa_bridge(const mbedtls_pk_context *ctx,
 
     TEST_EQUAL(mbedtls_pk_get_psa_attributes(ctx, usage_flag, &attributes), 0);
     TEST_EQUAL(mbedtls_pk_import_into_psa(ctx, &attributes, &psa_key), 0);
+
     psa_algorithm_t exercise_usage = psa_get_key_usage_flags(&attributes);
     psa_algorithm_t exercise_alg = psa_get_key_algorithm(&attributes);
-    TEST_ASSERT(mbedtls_test_psa_exercise_key(psa_key,
-                                              exercise_usage, exercise_alg));
+    if (mbedtls_test_can_exercise_psa_algorithm(exercise_alg)) {
+        TEST_ASSERT(mbedtls_test_psa_exercise_key(psa_key,
+                                                  exercise_usage,
+                                                  exercise_alg));
+    }
 
     mbedtls_test_set_step((unsigned long) -1);
     ok = 1;