boot_serial: Fix SMP echo sending incomplete CBOR container

In case when echo string would be too long to fit into buffer,
there would be no space left for container termination.
Due to lack of error checking such non-terminated container would
be sent out, where error response should be sent out instead.
Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

boot/boot_serial/src/boot_serial.c

@@ -104,6 +104,14 @@ static cbor_state_t cbor_state = {
     .backups = &dummy_backups
 };
 
+void reset_cbor_state(void)
+{
+    cbor_state.payload_mut = (uint8_t *)bs_obuf;
+    cbor_state.payload_end = (const uint8_t *)bs_obuf
+                             + sizeof(bs_obuf);
+    cbor_state.elem_count = 0;
+}
+
 /**
  * Function that processes MGMT_GROUP_ID_PERUSER mcumgr group and may be
  * used to process any groups that have not been processed by generic boot
@@ -470,6 +478,20 @@ out:
 #endif //#ifdef MCUBOOT_ENC_IMAGES
 }
 
+/*
+ * Send rc code only.
+ */
+static void
+bs_rc_rsp(int rc_code)
+{
+    map_start_encode(&cbor_state, 10);
+    tstrx_put(&cbor_state, "rc");
+    uintx32_put(&cbor_state, rc_code);
+    map_end_encode(&cbor_state, 10);
+    boot_serial_output();
+}
+
+
 #ifdef MCUBOOT_BOOT_MGMT_ECHO
 static bool
 decode_echo(cbor_state_t *state, cbor_string_type_t *result)
@@ -495,26 +517,16 @@ bs_echo(char *buf, int len)
     if (entry_function((const uint8_t *)buf, len, str, &bsstrdecoded, (void *)decode_echo, 1, 2)) {
         map_start_encode(&cbor_state, 10);
         tstrx_put(&cbor_state, "r");
-        tstrx_encode(&cbor_state, &str[1]);
-        map_end_encode(&cbor_state, 10);
-        boot_serial_output();
+        if (tstrx_encode(&cbor_state, &str[1]) && map_end_encode(&cbor_state, 10)) {
+            boot_serial_output();
+        } else {
+            reset_cbor_state();
+            bs_rc_rsp(MGMT_ERR_ENOMEM);
+        }
     }
 }
 #endif
 
-/*
- * Send rc code only.
- */
-static void
-bs_rc_rsp(int rc_code)
-{
-    map_start_encode(&cbor_state, 10);
-    tstrx_put(&cbor_state, "rc");
-    uintx32_put(&cbor_state, rc_code);
-    map_end_encode(&cbor_state, 10);
-    boot_serial_output();
-}
-
 /*
  * Reset, and (presumably) boot to newly uploaded image. Flush console
  * before restarting.
@@ -558,9 +570,7 @@ boot_serial_input(char *buf, int len)
     buf += sizeof(*hdr);
     len -= sizeof(*hdr);
 
-    cbor_state.payload_mut = (uint8_t *)bs_obuf;
-    cbor_state.payload_end = (const uint8_t *)bs_obuf
-                             + sizeof(bs_obuf);
+    reset_cbor_state();
 
     /*
      * Limited support for commands.

boot/boot_serial/src/boot_serial_priv.h

@@ -38,6 +38,7 @@ extern "C" {
  */
 #define MGMT_ERR_OK             0
 #define MGMT_ERR_EUNKNOWN       1
+#define MGMT_ERR_ENOMEM         2
 #define MGMT_ERR_EINVAL         3
 #define MGMT_ERR_ENOTSUP        8