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>

networking/udhcp/common.c

@@ -29,9 +29,9 @@ const struct dhcp_optflag dhcp_optflags[] = {
 //	{ OPTION_IP | OPTION_LIST                 , 0x07 }, /* DHCP_LOG_SERVER    */
 //	{ OPTION_IP | OPTION_LIST                 , 0x08 }, /* DHCP_COOKIE_SERVER */
 	{ OPTION_IP | OPTION_LIST                 , 0x09 }, /* DHCP_LPR_SERVER    */
-	{ OPTION_STRING               | OPTION_REQ, 0x0c }, /* DHCP_HOST_NAME     */
+	{ OPTION_STRING_HOST          | OPTION_REQ, 0x0c }, /* DHCP_HOST_NAME     */
 	{ OPTION_U16                              , 0x0d }, /* DHCP_BOOT_SIZE     */
-	{ OPTION_STRING               | OPTION_REQ, 0x0f }, /* DHCP_DOMAIN_NAME   */
+	{ OPTION_STRING_HOST          | OPTION_REQ, 0x0f }, /* DHCP_DOMAIN_NAME   */
 	{ OPTION_IP                               , 0x10 }, /* DHCP_SWAP_SERVER   */
 	{ OPTION_STRING                           , 0x11 }, /* DHCP_ROOT_PATH     */
 	{ OPTION_U8                               , 0x17 }, /* DHCP_IP_TTL        */
@@ -41,7 +41,7 @@ const struct dhcp_optflag dhcp_optflags[] = {
 //server would let us know anyway?
 	{ OPTION_IP                   | OPTION_REQ, 0x1c }, /* DHCP_BROADCAST     */
 	{ OPTION_IP_PAIR | OPTION_LIST            , 0x21 }, /* DHCP_ROUTES        */
-	{ OPTION_STRING                           , 0x28 }, /* DHCP_NIS_DOMAIN    */
+	{ OPTION_STRING_HOST                      , 0x28 }, /* DHCP_NIS_DOMAIN    */
 	{ OPTION_IP | OPTION_LIST                 , 0x29 }, /* DHCP_NIS_SERVER    */
 	{ OPTION_IP | OPTION_LIST     | OPTION_REQ, 0x2a }, /* DHCP_NTP_SERVER    */
 	{ OPTION_IP | OPTION_LIST                 , 0x2c }, /* DHCP_WINS_SERVER   */
@@ -49,7 +49,7 @@ const struct dhcp_optflag dhcp_optflags[] = {
 	{ OPTION_IP                               , 0x36 }, /* DHCP_SERVER_ID     */
 	{ OPTION_STRING                           , 0x38 }, /* DHCP_ERR_MESSAGE   */
 //TODO: must be combined with 'sname' and 'file' handling:
-	{ OPTION_STRING                           , 0x42 }, /* DHCP_TFTP_SERVER_NAME */
+	{ OPTION_STRING_HOST                      , 0x42 }, /* DHCP_TFTP_SERVER_NAME */
 	{ OPTION_STRING                           , 0x43 }, /* DHCP_BOOT_FILE     */
 //TODO: not a string, but a set of LASCII strings:
 //	{ OPTION_STRING                           , 0x4D }, /* DHCP_USER_CLASS    */
@@ -148,6 +148,7 @@ const uint8_t dhcp_option_lengths[] ALIGN1 = {
 	[OPTION_IP_PAIR] = 8,
 //	[OPTION_BOOLEAN] = 1,
 	[OPTION_STRING] =  1,  /* ignored by udhcp_str2optset */
+	[OPTION_STRING_HOST] = 1,  /* ignored by udhcp_str2optset */
 #if ENABLE_FEATURE_UDHCP_RFC3397
 	[OPTION_DNS_STRING] = 1,  /* ignored by both udhcp_str2optset and xmalloc_optname_optval */
 	[OPTION_SIP_SERVERS] = 1,
@@ -417,7 +418,9 @@ static NOINLINE void attach_option(
 			/* actually 255 is ok too, but adding a space can overlow it */
 
 			existing->data = xrealloc(existing->data, OPT_DATA + 1 + old_len + length);
-			if ((optflag->flags & OPTION_TYPE_MASK) == OPTION_STRING) {
+			if ((optflag->flags & OPTION_TYPE_MASK) == OPTION_STRING
+			 || (optflag->flags & OPTION_TYPE_MASK) == OPTION_STRING_HOST
+			) {
 				/* add space separator between STRING options in a list */
 				existing->data[OPT_DATA + old_len] = ' ';
 				old_len++;
@@ -481,6 +484,7 @@ int FAST_FUNC udhcp_str2optset(const char *const_str, void *arg)
 				retval = udhcp_str2nip(val, buffer + 4);
 			break;
 		case OPTION_STRING:
+		case OPTION_STRING_HOST:
 #if ENABLE_FEATURE_UDHCP_RFC3397
 		case OPTION_DNS_STRING:
 #endif

networking/udhcp/common.h

@@ -80,6 +80,9 @@ enum {
 	OPTION_IP = 1,
 	OPTION_IP_PAIR,
 	OPTION_STRING,
+	/* Opts of STRING_HOST type will be sanitized before they are passed
+	 * to udhcpc script's environment: */
+	OPTION_STRING_HOST,
 //	OPTION_BOOLEAN,
 	OPTION_U8,
 	OPTION_U16,

networking/udhcp/dhcpc.c

@@ -135,6 +135,63 @@ static int mton(uint32_t mask)
 	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 @@ static NOINLINE char *xmalloc_optname_optval(uint8_t *option, const struct dhcp_
 		 * 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 @@ static char **fill_envp(struct dhcp_packet *packet)
 	/* +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) {