Interpret --key and --cert option argument as URI

OpenSSL 3 has providers which can load keys and certificates
from various key stores and HSMs using a provider-specific URI.
While certificates are generally exportable, and some providers
support a PEM file that acts as a proxy for non-exportable private
keys, not all providers are expected to do so. A generic capability
to read keys and certificates from URIs appears useful.

This patch does this by extending the scope of the argument for
"--key" and "--cert" options to include URIs. Many of OpenSSL 3
utilities also work the same way: e.g., the "-in" option for
"openssl pkey" or "openssl x509" could be a filename or URI.
Other applications have started emulating this behaviour:
e.g., pkcs11: URI works as an alternative to a file name for
certificates and keys in apache. Even for files, this has a nice
side effect that non-PEM files get transparently parsed. E.g., a
pkcs12 file could be used in place of a PEM file without needing
any extra options.

This is backward compatible as OpenSSL falls back to treating URIs
with no scheme or unrecognized scheme as file names.

Parsing of inlined keys and certificates is unchanged (those
should be in PEM format).

Specification of URIs that OpenSSL accepts depends on the
providers that support them. Some are standard URIs such as
"file:/path", but providers may support non-standard URIs
with arbitrary scheme names. OpenSSL by itself recognizes
only file URI.  However, the implementation is agnostic to the
URI specification as parsing is done by the provider that supports
the URI. A new URI gets automatically recognized when the provider
that supports it is loaded.

Below are some usage examples:

Relative or absolute path to a file or as a URI "file:/absolute/path":

   --key mykey.pem      (same as what is currently supported)
   --key file:/path/to/mykey.pem
   --cert file:/path/to/mycert.pem

Other file types supported by OpenSSL would also work:

   --key client.p12
   --cert client.p12

pkcs11-provider supports "pkcs11:" URI (RFC 7512):

   --key pkcs11:token=Foo;id=%01
   --cert pkcs11:token=Foo;id=%01

tpm2-provider recognizes a custom URI "handle:<hex>":

   --key handle:0x81000000

These examples assume that required providers, if any, are loaded
and configured.

v2: same as PR 591 but with the fixup commit that addresses review comments is squashed.

Change-Id: I82b32d5ab472926e7889a5f4a90caba14231879a
Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240906103734.36633-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29075.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

doc/man-sections/tls-options.rst

@@ -85,10 +85,17 @@ certificates and keys: https://github.com/OpenVPN/easy-rsa
   OpenVPN will log the usual warning in the logs if the relevant CRL is
   missing, but the connection will be allowed.
 
---cert file
+--cert file|uri
-  Local peer's signed certificate in .pem format -- must be signed by a
+  Local peer's signed certificate in .pem format or as a URI -- must be
-  certificate authority whose certificate is in ``--ca file``. Each peer
+  signed by a certificate authority whose certificate is in ``--ca file``
-  in an OpenVPN link running in TLS mode should have its own certificate
+  in the peer configuration. URI is supported only when built with
+  OpenSSL 3.0 or later and any required providers are loaded. Types
+  of URIs supported and their syntax depends on providers. OpenSSL has
+  internal support for "file:/absolute/path" URI in which case the scheme
+  "file:" is optional, and any file format recognized by OpenSSL (e.g., PEM,
+  PKCS12) is supported. PKCS#11 URI (RFC 7512) is supported by pkcs11-provider.
+
+  Each peer in an OpenVPN link running in TLS mode should have its own certificate
   and private key file. In addition, each certificate should have been
   signed by the key of a certificate authority whose public key resides in
   the ``--ca`` certificate authority file. You can easily make your own
@@ -203,10 +210,11 @@ certificates and keys: https://github.com/OpenVPN/easy-rsa
   The ``--hand-window`` parameter also controls the amount of time that
   the OpenVPN client repeats the pull request until it times out.
 
---key file
+--key file|uri
-  Local peer's private key in .pem format. Use the private key which was
+  Local peer's private key in .pem format or a URI. Use the private key
-  generated when you built your peer's certificate (see ``--cert file``
+  which was generated when you built your peer's certificate (see
-  above).
+  ``--cert file`` above). URI is supported only when built with OpenSSL 3.0
+  or later and any required providers are loaded. (See `--cert` for more details).
 
 --pkcs12 file
   Specify a PKCS #12 file containing local private key, local certificate,

src/openvpn/options.c

@@ -567,10 +567,10 @@ static const char usage_message[] =
     "--dh file       : File containing Diffie Hellman parameters\n"
     "                  in .pem format (for --tls-server only).\n"
     "                  Use \"openssl dhparam -out dh1024.pem 1024\" to generate.\n"
-    "--cert file     : Local certificate in .pem format -- must be signed\n"
+    "--cert file     : Local certificate in .pem format or a URI -- must be signed\n"
-    "                  by a Certificate Authority in --ca file.\n"
+    "                  by a Certificate Authority in --ca file used by the peer.\n"
     "--extra-certs file : one or more PEM certs that complete the cert chain.\n"
-    "--key file      : Local private key in .pem format.\n"
+    "--key file      : Local private key in .pem format or a URI.\n"
     "--tls-version-min <version> ['or-highest'] : sets the minimum TLS version we\n"
     "    will accept from the peer.  If version is unrecognized and 'or-highest'\n"
     "    is specified, require max TLS version supported by SSL implementation.\n"
@@ -3851,6 +3851,7 @@ options_postprocess_mutate(struct options *o, struct env_set *es)
 #define CHKACC_FILEXSTWR (1<<2)  /** If file exists, is it writable? */
 #define CHKACC_ACPTSTDIN (1<<3)  /** If filename is stdin, it's allowed and "exists" */
 #define CHKACC_PRIVATE (1<<4)    /** Warn if this (private) file is group/others accessible */
+#define CHKACC_ACCEPT_URI (1<<5) /** If filename is a URI, no check is done unless it starts with file: */
 
 static bool
 check_file_access(const int type, const char *file, const int mode, const char *opt)
@@ -3871,6 +3872,21 @@ check_file_access(const int type, const char *file, const int mode, const char *
         return false;
     }
 
+    /* file name is a URI if its first segment  has ":" (i.e., before any "/")
+     * Then no checks done if CHKACC_ACCEPT_URI is set and the URI does not start with "file:"
+     */
+    if ((type & CHKACC_ACCEPT_URI) && strchr(file, ':'))
+    {
+        if (!strncmp(file, "file:", 5))
+        {
+            file += 5;
+        }
+        else if (!strchr(file, '/') || strchr(file, '/') > strchr(file, ':'))
+        {
+            return false;
+        }
+    }
+
     /* Is the directory path leading to the given file accessible? */
     if (type & CHKACC_DIRPATH)
     {
@@ -4069,7 +4085,7 @@ options_postprocess_filechecks(struct options *options)
     errs |= check_file_access_chroot(options->chroot_dir, CHKACC_FILE,
                                      options->ca_path, R_OK, "--capath");
 
-    errs |= check_file_access_inline(options->cert_file_inline, CHKACC_FILE,
+    errs |= check_file_access_inline(options->cert_file_inline, CHKACC_FILE|CHKACC_ACCEPT_URI,
                                      options->cert_file, R_OK, "--cert");
 
     errs |= check_file_access_inline(options->extra_certs_file, CHKACC_FILE,
@@ -4079,7 +4095,7 @@ options_postprocess_filechecks(struct options *options)
     if (!(options->management_flags & MF_EXTERNAL_KEY))
     {
         errs |= check_file_access_inline(options->priv_key_file_inline,
-                                         CHKACC_FILE|CHKACC_PRIVATE,
+                                         CHKACC_FILE|CHKACC_PRIVATE|CHKACC_ACCEPT_URI,
                                          options->priv_key_file, R_OK, "--key");
     }
 

src/openvpn/ssl_openssl.c

@@ -65,6 +65,12 @@
 #include <openssl/ec.h>
 #endif
 
+#if OPENSSL_VERSION_NUMBER >= 0x30000000L
+#define HAVE_OPENSSL_STORE_API
+#include <openssl/ui.h>
+#include <openssl/store.h>
+#endif
+
 #if defined(_MSC_VER) && !defined(_M_ARM64)
 #include <openssl/applink.c>
 #endif
@@ -768,6 +774,111 @@ tls_ctx_load_ecdh_params(struct tls_root_ctx *ctx, const char *curve_name)
 #endif /* OPENSSL_NO_EC */
 }
 
+#if defined(HAVE_OPENSSL_STORE_API)
+/**
+ * A wrapper for pem_password_callback for use with OpenSSL UI_METHOD.
+ */
+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)
+    {
+        const char *prompt = UI_get0_output_string(uis);
+
+        /* If pkcs#11 Use custom prompt similar to pkcs11-helper */
+        if (strstr(prompt, "PKCS#11"))
+        {
+            struct user_pass up;
+            get_user_pass(&up, NULL, "PKCS#11 token", GET_USER_PASS_MANAGEMENT|GET_USER_PASS_PASSWORD_ONLY);
+            UI_set_result(ui, uis, up.password);
+            purge_user_pass(&up, true);
+        }
+        else /* use our generic 'Private Key' passphrase callback */
+        {
+            char password[64];
+            pem_password_cb *cb = SSL_CTX_get_default_passwd_cb(ctx);
+            void *d = SSL_CTX_get_default_passwd_cb_userdata(ctx);
+
+            cb(password, sizeof(password), 0, d);
+            UI_set_result(ui, uis, password);
+            secure_memzero(password, sizeof(password));
+        }
+
+        return 1;
+    }
+    return 0;
+}
+#endif /* defined(HAVE_OPENSSL_STORE_API) */
+
+/**
+ * Load private key from OSSL_STORE URI or file
+ * uri : URI of object or filename
+ * ssl_ctx : SSL_CTX for UI prompt
+ *
+ * Return a pointer to the key or NULL if not found.
+ * Caller must free the key after use.
+ */
+static void *
+load_pkey_from_uri(const char *uri, SSL_CTX *ssl_ctx)
+{
+    EVP_PKEY *pkey = NULL;
+
+#if !defined(HAVE_OPENSSL_STORE_API)
+
+    /* Treat the uri as file name */
+    BIO *in = BIO_new_file(uri, "r");
+    if (!in)
+    {
+        return NULL;
+    }
+    pkey = PEM_read_bio_PrivateKey(in, NULL,
+                                   SSL_CTX_get_default_passwd_cb(ssl_ctx),
+                                   SSL_CTX_get_default_passwd_cb_userdata(ssl_ctx));
+    BIO_free(in);
+
+#else /* defined(HAVE_OPENSSL_STORE_API) */
+
+    OSSL_STORE_CTX *store_ctx = NULL;
+    OSSL_STORE_INFO *info = NULL;
+
+    UI_METHOD *ui_method = UI_create_method("openvpn");
+    if (!ui_method)
+    {
+        msg(M_WARN, "OpenSSL UI creation failed");
+        return NULL;
+    }
+    UI_method_set_reader(ui_method, ui_reader);
+
+    store_ctx = OSSL_STORE_open_ex(uri, tls_libctx, NULL, ui_method, ssl_ctx,
+                                   NULL, NULL, NULL);
+    if (!store_ctx)
+    {
+        goto end;
+    }
+    if (OSSL_STORE_expect(store_ctx, OSSL_STORE_INFO_PKEY) != 1)
+    {
+        goto end;
+    }
+    info = OSSL_STORE_load(store_ctx);
+    if (!info)
+    {
+        goto end;
+    }
+    pkey = OSSL_STORE_INFO_get1_PKEY(info);
+    OSSL_STORE_INFO_free(info);
+    msg(D_TLS_DEBUG_MED, "Found pkey in store using URI: %s", uri);
+
+end:
+    OSSL_STORE_close(store_ctx);
+    UI_destroy_method(ui_method);
+
+#endif /* defined(HAVE_OPENSSL_STORE_API) */
+
+    return pkey;
+}
+
 int
 tls_ctx_load_pkcs12(struct tls_root_ctx *ctx, const char *pkcs12_file,
                     bool pkcs12_file_inline, bool load_ca_file)
@@ -945,8 +1056,102 @@ tls_ctx_add_extra_certs(struct tls_root_ctx *ctx, BIO *bio, bool optional)
     }
 }
 
-void
+static bool
-tls_ctx_load_cert_file(struct tls_root_ctx *ctx, const char *cert_file,
+cert_uri_supported(void)
+{
+#if defined(HAVE_OPENSSL_STORE_API)
+    return 1;
+#else
+    return 0;
+#endif
+}
+
+static void
+tls_ctx_load_cert_uri(struct tls_root_ctx *tls_ctx, const char *uri)
+{
+#if defined(HAVE_OPENSSL_STORE_API)
+    X509 *x = NULL;
+    int ret = 0;
+    OSSL_STORE_CTX *store_ctx = NULL;
+    OSSL_STORE_INFO *info = NULL;
+
+    ASSERT(NULL != tls_ctx);
+
+    UI_METHOD *ui_method = UI_create_method("openvpn");
+    if (!ui_method)
+    {
+        msg(M_WARN, "OpenSSL UI method creation failed");
+        goto end;
+    }
+    UI_method_set_reader(ui_method, ui_reader);
+
+    store_ctx = OSSL_STORE_open_ex(uri, tls_libctx, NULL, ui_method, tls_ctx->ctx,
+                                   NULL, NULL, NULL);
+    if (!store_ctx)
+    {
+        goto end;
+    }
+    if (OSSL_STORE_expect(store_ctx, OSSL_STORE_INFO_CERT) != 1)
+    {
+        goto end;
+    }
+
+    info = OSSL_STORE_load(store_ctx);
+    if (!info)
+    {
+        goto end;
+    }
+
+    x = OSSL_STORE_INFO_get0_CERT(info);
+    if (x == NULL)
+    {
+        goto end;
+    }
+    msg(D_TLS_DEBUG_MED, "Found cert in store using URI: %s", uri);
+
+    ret = SSL_CTX_use_certificate(tls_ctx->ctx, x);
+    if (!ret)
+    {
+        goto end;
+    }
+    OSSL_STORE_INFO_free(info);
+
+    /* iterate through the store and add extra certificates if any to the chain */
+    info = OSSL_STORE_load(store_ctx);
+    while (info && !OSSL_STORE_eof(store_ctx))
+    {
+        x = OSSL_STORE_INFO_get1_CERT(info);
+        if (x && SSL_CTX_add_extra_chain_cert(tls_ctx->ctx, x) != 1)
+        {
+            X509_free(x);
+            crypto_msg(M_FATAL, "Error adding extra certificate");
+            break;
+        }
+        OSSL_STORE_INFO_free(info);
+        info = OSSL_STORE_load(store_ctx);
+    }
+
+end:
+    if (!ret)
+    {
+        crypto_print_openssl_errors(M_WARN);
+        crypto_msg(M_FATAL, "Cannot load certificate from URI <%s>", uri);
+    }
+    else
+    {
+        crypto_print_openssl_errors(M_DEBUG);
+    }
+
+    UI_destroy_method(ui_method);
+    OSSL_STORE_INFO_free(info);
+    OSSL_STORE_close(store_ctx);
+#else /* defined(HAVE_OPENSSL_STORE_API */
+    ASSERT(0);
+#endif /* defined(HAVE_OPENSSL_STORE_API */
+}
+
+static void
+tls_ctx_load_cert_pem_file(struct tls_root_ctx *ctx, const char *cert_file,
                            bool cert_file_inline)
 {
     BIO *in = NULL;
@@ -961,7 +1166,7 @@ tls_ctx_load_cert_file(struct tls_root_ctx *ctx, const char *cert_file,
     }
     else
     {
-        in = BIO_new_file(cert_file, "r");
+        in = BIO_new_file((char *) cert_file, "r");
     }
 
     if (in == NULL)
@@ -1007,6 +1212,20 @@ end:
     X509_free(x);
 }
 
+void
+tls_ctx_load_cert_file(struct tls_root_ctx *ctx, const char *cert_file,
+                       bool cert_file_inline)
+{
+    if (cert_uri_supported() && !cert_file_inline)
+    {
+        tls_ctx_load_cert_uri(ctx, cert_file);
+    }
+    else
+    {
+        tls_ctx_load_cert_pem_file(ctx, cert_file, cert_file_inline);
+    }
+}
+
 int
 tls_ctx_load_priv_file(struct tls_root_ctx *ctx, const char *priv_key_file,
                        bool priv_key_file_inline)
@@ -1023,20 +1242,18 @@ tls_ctx_load_priv_file(struct tls_root_ctx *ctx, const char *priv_key_file,
     if (priv_key_file_inline)
     {
         in = BIO_new_mem_buf((char *) priv_key_file, -1);
-    }
+        if (in == NULL)
-    else
-    {
-        in = BIO_new_file(priv_key_file, "r");
-    }
-
-    if (!in)
         {
             goto end;
         }
-
         pkey = PEM_read_bio_PrivateKey(in, NULL,
                                        SSL_CTX_get_default_passwd_cb(ctx->ctx),
                                        SSL_CTX_get_default_passwd_cb_userdata(ctx->ctx));
+    }
+    else
+    {
+        pkey = load_pkey_from_uri(priv_key_file, ssl_ctx);
+    }
 
     if (!pkey || !SSL_CTX_use_PrivateKey(ssl_ctx, pkey))
     {

tests/unit_tests/openvpn/test_ssl.c

@@ -66,6 +66,18 @@ throw_signal(const int signum)
 }
 #endif
 
+/* stubs for some unused functions instead of pulling in too many dependencies */
+bool
+get_user_pass_cr(struct user_pass *up, const char *auth_file, const char *prefix,
+                 const unsigned int flags, const char *auth_challenge)
+{
+    return false;
+}
+void
+purge_user_pass(struct user_pass *up, bool force)
+{
+    return;
+}
 
 const char *unittest_cert = "-----BEGIN CERTIFICATE-----\n"
                             "MIIBuTCCAUCgAwIBAgIUTLtjSBzx53qZRvZ6Ur7D9kgoOHkwCgYIKoZIzj0EAwIw\n"