openssl: add engine method for loading the key

As well as doing crypto acceleration, engines can also be used to load
key files.  If the engine is set, and the private key loading fails
for bio methods, this patch makes openvpn try to get the engine to
load the key.  If that succeeds, we end up using an engine based key.
This can be used with the openssl tpm engines to make openvpn use a
TPM wrapped key file.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20200528225920.6983-2-James.Bottomley@HansenPartnership.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19937.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

Author: James Bottomley

src/openvpn/crypto_openssl.c

@@ -63,6 +63,7 @@
 #endif
 
 #if HAVE_OPENSSL_ENGINE
+#include <openssl/ui.h>
 #include <openssl/engine.h>
 
 static bool engine_initialized = false; /* GLOBAL */
@@ -1070,4 +1071,59 @@ memcmp_constant_time(const void *a, const void *b, size_t size)
 {
     return CRYPTO_memcmp(a, b, size);
 }
+
+#if HAVE_OPENSSL_ENGINE
+static int
+ui_reader(UI *ui, UI_STRING *uis)
+{
+    SSL_CTX *ctx = UI_get0_user_data(ui);
+
+    if (UI_get_string_type(uis) == UIT_PROMPT) {
+        pem_password_cb *cb = SSL_CTX_get_default_passwd_cb(ctx);
+        void *d = SSL_CTX_get_default_passwd_cb_userdata(ctx);
+        char password[64];
+
+        cb(password, sizeof(password), 0, d);
+        UI_set_result(ui, uis, password);
+
+        return 1;
+    }
+    return 0;
+}
+#endif
+
+EVP_PKEY *
+engine_load_key(const char *file, SSL_CTX *ctx)
+{
+#if HAVE_OPENSSL_ENGINE
+    UI_METHOD *ui;
+    EVP_PKEY *pkey;
+
+    if (!engine_persist)
+        return NULL;
+
+    /* this will print out the error from BIO_read */
+    crypto_msg(M_INFO, "PEM_read_bio failed, now trying engine method to load private key");
+
+    ui = UI_create_method("openvpn");
+    if (!ui) {
+	crypto_msg(M_FATAL, "Engine UI creation failed");
+        return NULL;
+    }
+
+    UI_method_set_reader(ui, ui_reader);
+
+    ENGINE_init(engine_persist);
+    pkey = ENGINE_load_private_key(engine_persist, file, ui, ctx);
+    ENGINE_finish(engine_persist);
+    if (!pkey)
+	crypto_msg(M_FATAL, "Engine could not load key file");
+
+    UI_destroy_method(ui);
+    return pkey;
+#else
+    return NULL;
+#endif
+}
+
 #endif /* ENABLE_CRYPTO_OPENSSL */

src/openvpn/crypto_openssl.h

@@ -107,4 +107,16 @@ cipher_kt_var_key_size(const cipher_kt_t *cipher)
     return EVP_CIPHER_flags(cipher) & EVP_CIPH_VARIABLE_LENGTH;
 }
 
+/**
+ * Load a key file from an engine
+ *
+ * @param file	The engine file to load
+ * @param ui	The UI method for the password prompt
+ * @param data	The data to pass to the UI method
+ *
+ * @return	The private key if successful or NULL if not
+ */
+EVP_PKEY *
+engine_load_key(const char *file, SSL_CTX *ctx);
+
 #endif /* CRYPTO_OPENSSL_H_ */

src/openvpn/ssl_openssl.c

@@ -1020,6 +1020,11 @@ tls_ctx_load_priv_file(struct tls_root_ctx *ctx, const char *priv_key_file,
     pkey = PEM_read_bio_PrivateKey(in, NULL,
                                    SSL_CTX_get_default_passwd_cb(ctx->ctx),
                                    SSL_CTX_get_default_passwd_cb_userdata(ctx->ctx));
+    if (!pkey)
+    {
+        pkey = engine_load_key(priv_key_file, ctx->ctx);
+    }
+
     if (!pkey || !SSL_CTX_use_PrivateKey(ssl_ctx, pkey))
     {
 #ifdef ENABLE_MANAGEMENT