udhcpc: sanitize hostnames in incoming packets. Closes 3979.
The following options are replaced with string "bad" if they
contain malformed hostname:
HOST_NAME, DOMAIN_NAME, NIS_DOMAIN, TFTP_SERVER_NAME
function old new delta
xmalloc_optname_optval 850 888 +38
attach_option 440 443 +3
len_of_option_as_string 13 14 +1
dhcp_option_lengths 13 14 +1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/0 up/down: 43/0) Total: 43 bytes
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index 945600c..2f2016c 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -135,6 +135,63 @@
return i;
}
+/* Check if a given label represents a valid DNS label
+ * Return pointer to the first character after the label upon success,
+ * NULL otherwise.
+ * See RFC1035, 2.3.1
+ */
+/* We don't need to be particularly anal. For example, allowing _, hyphen
+ * at the end, or leading and trailing dots would be ok, since it
+ * can't be used for attacks. (Leading hyphen can be, if someone uses
+ * cmd "$hostname"
+ * in the script: then hostname may be treated as an option)
+ */
+static const char *valid_domain_label(const char *label)
+{
+ unsigned char ch;
+ unsigned pos = 0;
+
+ for (;;) {
+ ch = *label;
+ if ((ch|0x20) < 'a' || (ch|0x20) > 'z') {
+ if (pos == 0) {
+ /* label must begin with letter */
+ return NULL;
+ }
+ if (ch < '0' || ch > '9') {
+ if (ch == '\0' || ch == '.')
+ return label;
+ /* DNS allows only '-', but we are more permissive */
+ if (ch != '-' && ch != '_')
+ return NULL;
+ }
+ }
+ label++;
+ pos++;
+ //Do we want this?
+ //if (pos > 63) /* NS_MAXLABEL; labels must be 63 chars or less */
+ // return NULL;
+ }
+}
+
+/* Check if a given name represents a valid DNS name */
+/* See RFC1035, 2.3.1 */
+static int good_hostname(const char *name)
+{
+ //const char *start = name;
+
+ for (;;) {
+ name = valid_domain_label(name);
+ if (!name)
+ return 0;
+ if (!name[0])
+ return 1;
+ //Do we want this?
+ //return ((name - start) < 1025); /* NS_MAXDNAME */
+ name++;
+ }
+}
+
/* Create "opt_name=opt_value" string */
static NOINLINE char *xmalloc_optname_optval(uint8_t *option, const struct dhcp_optflag *optflag, const char *opt_name)
{
@@ -187,8 +244,11 @@
* the case of list of options.
*/
case OPTION_STRING:
+ case OPTION_STRING_HOST:
memcpy(dest, option, len);
dest[len] = '\0';
+ if (type == OPTION_STRING_HOST && !good_hostname(dest))
+ safe_strncpy(dest, "bad", len);
return ret;
case OPTION_STATIC_ROUTES: {
/* Option binary format:
@@ -368,6 +428,7 @@
/* +1 element for each option, +2 for subnet option: */
if (packet) {
/* note: do not search for "pad" (0) and "end" (255) options */
+//TODO: change logic to scan packet _once_
for (i = 1; i < 255; i++) {
temp = udhcp_get_option(packet, i);
if (temp) {