fix: Unsets global libcrypto rand (#4424)

tests/s2n_test.h

@@ -61,17 +61,12 @@ int test_count;
  * not initialise at the start of the test. Useful for tests that e.g spawn a
  * number of independent childs at the start of a unit test and where you want
  * each child to have its own independently initialised s2n.
- *
- * BEGIN_TEST() prints unit test information to stdout. But this often gets
- * buffered by the kernel and will then be flushed in each child spawned. The
- * result is a number of repeated messages being send to stdout and, in turn,
- * appear in the logs. At the moment, we think this is better than risking not
- * having any printing at all.
  */
 #define BEGIN_TEST_NO_INIT()                                        \
     do {                                                            \
         test_count = 0;                                             \
         fprintf(stdout, "Running %-50s ... ", __FILE__);            \
+        fflush(stdout);                                             \
         EXPECT_SUCCESS_WITHOUT_COUNT(s2n_in_unit_test_set(true));   \
         S2N_TEST_OPTIONALLY_ENABLE_FIPS_MODE();                     \
     } while(0)

tests/unit/s2n_random_test.c

@@ -792,6 +792,30 @@ static int s2n_random_rand_bytes_after_cleanup_cb(struct random_test_case *test_
     return S2N_SUCCESS;
 }
 
+static int s2n_random_rand_bytes_before_init(struct random_test_case *test_case)
+{
+#if S2N_LIBCRYPTO_SUPPORTS_CUSTOM_RAND
+    /* Calling RAND_bytes will set a global random method */
+    unsigned char rndbytes[16] = { 0 };
+    EXPECT_EQUAL(RAND_bytes(rndbytes, sizeof(rndbytes)), 1);
+    const RAND_METHOD *rand_method = RAND_get_rand_method();
+    EXPECT_NOT_NULL(rand_method);
+    EXPECT_NOT_EQUAL(rand_method->bytes, s2n_openssl_compat_rand);
+
+    EXPECT_SUCCESS(s2n_init());
+
+    /* The global random method is overridden after calling s2n_init() */
+    const RAND_METHOD *custom_rand_method = RAND_get_rand_method();
+    EXPECT_NOT_NULL(custom_rand_method);
+    EXPECT_EQUAL(custom_rand_method->bytes, s2n_openssl_compat_rand);
+
+    /* RAND_bytes is still successful */
+    EXPECT_EQUAL(RAND_bytes(rndbytes, sizeof(rndbytes)), 1);
+
+#endif
+    return S2N_SUCCESS;
+}
+
 static int s2n_random_invalid_urandom_fd_cb(struct random_test_case *test_case)
 {
     EXPECT_SUCCESS(s2n_disable_atexit());
@@ -862,6 +886,7 @@ struct random_test_case random_test_cases[] = {
     { "Test failure.", s2n_random_test_case_failure_cb, CLONE_TEST_DETERMINE_AT_RUNTIME, 1 },
     { "Test libcrypto's RAND engine is reset correctly after manual s2n_cleanup()", s2n_random_rand_bytes_after_cleanup_cb, CLONE_TEST_DETERMINE_AT_RUNTIME, EXIT_SUCCESS },
     { "Test getting entropy with an invalid file descriptor", s2n_random_invalid_urandom_fd_cb, CLONE_TEST_DETERMINE_AT_RUNTIME, EXIT_SUCCESS },
+    { "Test libcrypto's global RAND is unset after calling s2n_init()", s2n_random_rand_bytes_before_init, CLONE_TEST_DETERMINE_AT_RUNTIME, EXIT_SUCCESS },
 };
 
 int main(int argc, char **argv)

utils/s2n_random.c

@@ -553,6 +553,9 @@ S2N_RESULT s2n_rand_init(void)
         return S2N_RESULT_OK;
     }
 
+    /* Unset any existing random engine */
+    RESULT_GUARD_OSSL(RAND_set_rand_engine(NULL), S2N_ERR_OPEN_RANDOM);
+
     /* Create an engine */
     ENGINE *e = ENGINE_new();
 

utils/s2n_random.h

@@ -40,3 +40,4 @@ S2N_RESULT s2n_get_public_random_bytes_used(uint64_t *bytes_used);
 S2N_RESULT s2n_get_private_random_data(struct s2n_blob *blob);
 S2N_RESULT s2n_get_private_random_bytes_used(uint64_t *bytes_used);
 S2N_RESULT s2n_public_random(int64_t max, uint64_t *output);
+int s2n_openssl_compat_rand(unsigned char *buf, int num);