file: strengthen exec access control

Do not allow setting environment variables if there is a session as
there is no access control for environment variables and allowing
arbitrary data into the environment is unsafe. Do not leak arguments
through unchecked if the size of the buffer for access checking the
whole command line is exceeded. Adjust the maximum number of allowed
arguments so it matches the actual implementation.

Signed-off-by: Erik Karlsson <erik.karlsson@genexis.eu>

file.c

@@ -809,6 +809,9 @@ rpc_file_exec_run(const char *cmd, const struct blob_attr *sid,
 
 	struct rpc_file_exec_context *c;
 
+	if (sid && env)
+		return UBUS_STATUS_PERMISSION_DENIED;
+
 	cmd = rpc_file_exec_lookup(cmd);
 
 	if (!cmd)
@@ -824,7 +827,7 @@ rpc_file_exec_run(const char *cmd, const struct blob_attr *sid,
 		if (arg == NULL || strlen(executable) >= sizeof(cmdstr))
 			return UBUS_STATUS_PERMISSION_DENIED;
 
-		arglen = 0;
+		arglen = 2;
 		p = cmdstr + sprintf(cmdstr, "%s", executable);
 
 		blobmsg_for_each_attr(cur, arg, rem)
@@ -834,7 +837,7 @@ rpc_file_exec_run(const char *cmd, const struct blob_attr *sid,
 
 			if (arglen == 255 ||
 			    p + blobmsg_data_len(cur) >= cmdstr + sizeof(cmdstr))
-				break;
+				return UBUS_STATUS_PERMISSION_DENIED;
 
 			p += sprintf(p, " %s", blobmsg_get_string(cur));
 			arglen++;