rsa: Accept NULL OAEP label for backward compatibility

According to the manual page, EVP_PKEY_CTX_set0_rsa_oaep_label()
should accept NULL as the label argument, though the function
currently rejects it while setting the corresponding octet string
parameter with OSSL_PARAM_construct_octet_string, which expects
non-NULL input.  This adds a workaround to the caller for backward
compatibility.
Signed-off-by: NDaiki Ueno <dueno@redhat.com>
Reviewed-by: NDmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: NTomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22397)

crypto/rsa/rsa_lib.c

@@ -1086,6 +1086,12 @@ int EVP_PKEY_CTX_get_rsa_mgf1_md(EVP_PKEY_CTX *ctx, const EVP_MD **md)
 int EVP_PKEY_CTX_set0_rsa_oaep_label(EVP_PKEY_CTX *ctx, void *label, int llen)
 {
     OSSL_PARAM rsa_params[2], *p = rsa_params;
+    const char *empty = "";
+    /*
+     * Needed as we swap label with empty if it is NULL, and label is
+     * freed at the end of this function.
+     */
+    void *plabel = label;
     int ret;
 
     if (ctx == NULL || !EVP_PKEY_CTX_IS_ASYM_CIPHER_OP(ctx)) {
@@ -1098,9 +1104,13 @@ int EVP_PKEY_CTX_set0_rsa_oaep_label(EVP_PKEY_CTX *ctx, void *label, int llen)
     if (!EVP_PKEY_CTX_is_a(ctx, "RSA"))
         return -1;
 
+    /* Accept NULL for backward compatibility */
+    if (label == NULL && llen == 0)
+        plabel = (void *)empty;
+
     /* Cast away the const. This is read only so should be safe */
     *p++ = OSSL_PARAM_construct_octet_string(OSSL_ASYM_CIPHER_PARAM_OAEP_LABEL,
-                                             (void *)label, (size_t)llen);
+                                             (void *)plabel, (size_t)llen);
     *p++ = OSSL_PARAM_construct_end();
 
     ret = evp_pkey_ctx_set_params_strict(ctx, rsa_params);

test/evp_extra_test.c

@@ -3046,6 +3046,36 @@ static int test_RSA_OAEP_set_get_params(void)
     return ret;
 }
 
+/* https://github.com/openssl/openssl/issues/21288 */
+static int test_RSA_OAEP_set_null_label(void)
+{
+    int ret = 0;
+    EVP_PKEY *key = NULL;
+    EVP_PKEY_CTX *key_ctx = NULL;
+
+    if (!TEST_ptr(key = load_example_rsa_key())
+        || !TEST_ptr(key_ctx = EVP_PKEY_CTX_new_from_pkey(testctx, key, NULL))
+        || !TEST_true(EVP_PKEY_encrypt_init(key_ctx)))
+        goto err;
+
+    if (!TEST_true(EVP_PKEY_CTX_set_rsa_padding(key_ctx, RSA_PKCS1_OAEP_PADDING)))
+        goto err;
+
+    if (!TEST_true(EVP_PKEY_CTX_set0_rsa_oaep_label(key_ctx, OPENSSL_strdup("foo"), 0)))
+        goto err;
+
+    if (!TEST_true(EVP_PKEY_CTX_set0_rsa_oaep_label(key_ctx, NULL, 0)))
+        goto err;
+
+    ret = 1;
+
+ err:
+    EVP_PKEY_free(key);
+    EVP_PKEY_CTX_free(key_ctx);
+
+    return ret;
+}
+
 #if !defined(OPENSSL_NO_CHACHA) && !defined(OPENSSL_NO_POLY1305)
 static int test_decrypt_null_chunks(void)
 {
@@ -5101,6 +5131,7 @@ int setup_tests(void)
 #endif
     ADD_TEST(test_RSA_get_set_params);
     ADD_TEST(test_RSA_OAEP_set_get_params);
+    ADD_TEST(test_RSA_OAEP_set_null_label);
 #if !defined(OPENSSL_NO_CHACHA) && !defined(OPENSSL_NO_POLY1305)
     ADD_TEST(test_decrypt_null_chunks);
 #endif