tools: Ensure that kernelfile and kernelparams are null-terminated

Analogously to read_env(), ensure also when reading an environment from
a specified file that those statically sized strings are properly
terminated before accessing them. Prevents potential out-of-bounds read
accesses in bg_printenv or bg_setenv.

Addresses CVE-2023-39950

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

tools/bg_envtools.c

@@ -154,5 +154,10 @@ bool get_env(char *configfilepath, BG_ENVDATA *data)
 		VERBOSE(stderr,
 			"Error closing environment file after reading.\n");
 	};
+
+	/* enforce NULL-termination of strings */
+	data->kernelfile[ENV_STRING_LENGTH - 1] = 0;
+	data->kernelparams[ENV_STRING_LENGTH - 1] = 0;
+
 	return result;
 }