extmod/modtls_mbedtls: Implement cert verification callback for mbedtls.

This is a useful alternative to .getpeercert() when the certificate is not
stored to reduce RAM usage.

Signed-off-by: Felix Dörre <felix@dogcraft.de>

extmod/modtls_mbedtls.c

@@ -67,6 +67,7 @@ typedef struct _mp_obj_ssl_context_t {
     mbedtls_pk_context pkey;
     int authmode;
     int *ciphersuites;
+    mp_obj_t handler;
 } mp_obj_ssl_context_t;
 
 // This corresponds to an SSLSocket object.
@@ -188,6 +189,16 @@ STATIC void ssl_check_async_handshake_failure(mp_obj_ssl_socket_t *sslsock, int
     }
 }
 
+STATIC int ssl_sock_cert_verify(void *ptr, mbedtls_x509_crt *crt, int depth, uint32_t *flags) {
+    mp_obj_ssl_context_t *o = ptr;
+    if (o->handler == mp_const_none) {
+        return 0;
+    }
+    mp_obj_array_t cert;
+    mp_obj_memoryview_init(&cert, 'B', 0, crt->raw.len, crt->raw.p);
+    return mp_obj_get_int(mp_call_function_2(o->handler, MP_OBJ_FROM_PTR(&cert), MP_OBJ_NEW_SMALL_INT(depth)));
+}
+
 /******************************************************************************/
 // SSLContext type.
 
@@ -213,6 +224,7 @@ STATIC mp_obj_t ssl_context_make_new(const mp_obj_type_t *type_in, size_t n_args
     mbedtls_x509_crt_init(&self->cert);
     mbedtls_pk_init(&self->pkey);
     self->ciphersuites = NULL;
+    self->handler = mp_const_none;
 
     #ifdef MBEDTLS_DEBUG_C
     // Debug level (0-4) 1=warning, 2=info, 3=debug, 4=verbose
@@ -243,6 +255,7 @@ STATIC mp_obj_t ssl_context_make_new(const mp_obj_type_t *type_in, size_t n_args
         self->authmode = MBEDTLS_SSL_VERIFY_NONE;
     }
     mbedtls_ssl_conf_authmode(&self->conf, self->authmode);
+    mbedtls_ssl_conf_verify(&self->conf, &ssl_sock_cert_verify, self);
     mbedtls_ssl_conf_rng(&self->conf, mbedtls_ctr_drbg_random, &self->ctr_drbg);
     #ifdef MBEDTLS_DEBUG_C
     mbedtls_ssl_conf_dbg(&self->conf, mbedtls_debug, NULL);
@@ -257,6 +270,8 @@ STATIC void ssl_context_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
         // Load attribute.
         if (attr == MP_QSTR_verify_mode) {
             dest[0] = MP_OBJ_NEW_SMALL_INT(self->authmode);
+        } else if (attr == MP_QSTR_verify_callback) {
+            dest[0] = self->handler;
         } else {
             // Continue lookup in locals_dict.
             dest[1] = MP_OBJ_SENTINEL;
@@ -267,6 +282,9 @@ STATIC void ssl_context_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
             self->authmode = mp_obj_get_int(dest[1]);
             dest[0] = MP_OBJ_NULL;
             mbedtls_ssl_conf_authmode(&self->conf, self->authmode);
+        } else if (attr == MP_QSTR_verify_callback) {
+            dest[0] = MP_OBJ_NULL;
+            self->handler = dest[1];
         }
     }
 }

tests/multi_net/sslcontext_verify_callback.py

@@ -0,0 +1,68 @@
+# Test creating an SSL connection and getting the peer certificate.
+
+try:
+    import io
+    import os
+    import socket
+    import tls
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+PORT = 8000
+
+# These are test certificates. See tests/README.md for details.
+cert = cafile = "ec_cert.der"
+key = "ec_key.der"
+
+try:
+    with open(cafile, "rb") as f:
+        cadata = f.read()
+    with open(key, "rb") as f:
+        key = f.read()
+except OSError:
+    print("SKIP")
+    raise SystemExit
+
+
+def verify_callback(cert, depth):
+    print(cert.hex())
+    return 0
+
+
+# Server
+def instance0():
+    multitest.globals(IP=multitest.get_network_ip())
+    s = socket.socket()
+    s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+    s.bind(socket.getaddrinfo("0.0.0.0", PORT)[0][-1])
+    s.listen(1)
+    multitest.next()
+    s2, _ = s.accept()
+    server_ctx = tls.SSLContext(tls.PROTOCOL_TLS_SERVER)
+    server_ctx.load_cert_chain(cadata, key)
+    s2 = server_ctx.wrap_socket(s2, server_side=True)
+    print(s2.read(16))
+    s2.write(b"server to client")
+    s2.close()
+    s.close()
+
+
+# Client
+def instance1():
+    s_test = tls.SSLContext(tls.PROTOCOL_TLS_CLIENT)
+    if not hasattr(s_test, "verify_callback"):
+        print("SKIP")
+        raise SystemExit
+
+    multitest.next()
+    s = socket.socket()
+    s.connect(socket.getaddrinfo(IP, PORT)[0][-1])
+    client_ctx = tls.SSLContext(tls.PROTOCOL_TLS_CLIENT)
+    client_ctx.verify_mode = tls.CERT_REQUIRED
+    client_ctx.verify_callback = verify_callback
+    client_ctx.load_verify_locations(cadata)
+    s = client_ctx.wrap_socket(s, server_hostname="micropython.local")
+    s.write(b"client to server")
+    print(s.read(16))
+    s.close()

tests/multi_net/sslcontext_verify_callback.py.exp

@@ -0,0 +1,5 @@
+--- instance0 ---
+b'client to server'
+--- instance1 ---
+308201d330820179a00302010202144315a7cd8f69febe2640314e7c97d60a2523ad15300a06082a8648ce3d040302303f311a301806035504030c116d6963726f707974686f6e2e6c6f63616c31143012060355040a0c0b4d6963726f507974686f6e310b3009060355040613024155301e170d3234303131343034353335335a170d3235303131333034353335335a303f311a301806035504030c116d6963726f707974686f6e2e6c6f63616c31143012060355040a0c0b4d6963726f507974686f6e310b30090603550406130241553059301306072a8648ce3d020106082a8648ce3d0301070342000449b7f5fa687cb25a9464c397508149992f445c860bcf7002958eb4337636c6af840cd4c8cf3b96f2384860d8ae3ee3fa135dba051e8605e62bd871689c6af43ca3533051301d0603551d0e0416041441b3ae171d91e330411d8543ba45e0f2d5b2951b301f0603551d2304183016801441b3ae171d91e330411d8543ba45e0f2d5b2951b300f0603551d130101ff040530030101ff300a06082a8648ce3d04030203480030450220587f61c34739d6fab5802a674dcc54443ae9c87da374078c4ee1cd83f4ad1694022100cfc45dcf264888c6ba2c36e78bd27bb67856d7879a052dd7aa7ecf7215f7b992
+b'server to client'

tests/net_hosted/ssl_verify_callback.py

@@ -0,0 +1,37 @@
+# test ssl verify_callback
+
+import io
+import socket
+import tls
+
+
+def verify_callback(cert, depth):
+    print("verify_callback:", type(cert), len(cert) > 100, depth)
+    return 0
+
+
+def verify_callback_fail(cert, depth):
+    print("verify_callback_fail:", type(cert), len(cert) > 100, depth)
+    return 1
+
+
+def test(peer_addr):
+    context = tls.SSLContext(tls.PROTOCOL_TLS_CLIENT)
+    context.verify_mode = tls.CERT_OPTIONAL
+    context.verify_callback = verify_callback
+    s = socket.socket()
+    s.connect(peer_addr)
+    s = context.wrap_socket(s)
+    s.close()
+
+    context.verify_callback = verify_callback_fail
+    s = socket.socket()
+    s.connect(peer_addr)
+    try:
+        s = context.wrap_socket(s)
+    except OSError as e:
+        print(e.args[1])
+
+
+if __name__ == "__main__":
+    test(socket.getaddrinfo("micropython.org", 443)[0][-1])

tests/net_hosted/ssl_verify_callback.py.exp

@@ -0,0 +1,5 @@
+verify_callback: <class 'bytes'> True 2
+verify_callback: <class 'bytes'> True 1
+verify_callback: <class 'bytes'> True 0
+verify_callback_fail: <class 'bytes'> True 2
+MBEDTLS_ERR_ERROR_GENERIC_ERROR