udhcpc: check that 4-byte options are indeed 4-byte, closes 11506

function                                             old     new   delta
udhcp_get_option32                                     -      27     +27
udhcp_get_option                                     231     248     +17
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/0 up/down: 44/0)               Total: 44 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

4 files changed, 29 insertions, 6 deletions

diff --git a/networking/udhcp/common.c b/networking/udhcp/common.c
index e5fd74f..41b05b8 100644
--- a/networking/udhcp/common.c
+++ b/networking/udhcp/common.c
@@ -272,6 +272,15 @@ uint8_t* FAST_FUNC udhcp_get_option(struct dhcp_packet *packet, int code)
 			goto complain; /* complain and return NULL */
 
 		if (optionptr[OPT_CODE] == code) {
+			if (optionptr[OPT_LEN] == 0) {
+				/* So far no valid option with length 0 known.
+				 * Having this check means that searching
+				 * for DHCP_MESSAGE_TYPE need not worry
+				 * that returned pointer might be unsafe
+				 * to dereference.
+				 */
+				goto complain; /* complain and return NULL */
+			}
 			log_option("option found", optionptr);
 			return optionptr + OPT_DATA;
 		}
@@ -289,6 +298,16 @@ uint8_t* FAST_FUNC udhcp_get_option(struct dhcp_packet *packet, int code)
 	return NULL;
 }
 
+uint8_t* FAST_FUNC udhcp_get_option32(struct dhcp_packet *packet, int code)
+{
+	uint8_t *r = udhcp_get_option(packet, code);
+	if (r) {
+		if (r[-1] != 4)
+			r = NULL;
+	}
+	return r;
+}
+
 /* Return the position of the 'end' option (no bounds checking) */
 int FAST_FUNC udhcp_end_option(uint8_t *optionptr)
 {
diff --git a/networking/udhcp/common.h b/networking/udhcp/common.h
index 7ad603d..9511152 100644
--- a/networking/udhcp/common.h
+++ b/networking/udhcp/common.h
@@ -205,6 +205,10 @@ extern const uint8_t dhcp_option_lengths[] ALIGN1;
 unsigned FAST_FUNC udhcp_option_idx(const char *name, const char *option_strings);
 
 uint8_t *udhcp_get_option(struct dhcp_packet *packet, int code) FAST_FUNC;
+/* Same as above + ensures that option length is 4 bytes
+ * (returns NULL if size is different)
+ */
+uint8_t *udhcp_get_option32(struct dhcp_packet *packet, int code) FAST_FUNC;
 int udhcp_end_option(uint8_t *optionptr) FAST_FUNC;
 void udhcp_add_binary_option(struct dhcp_packet *packet, uint8_t *addopt) FAST_FUNC;
 #if ENABLE_UDHCPC || ENABLE_UDHCPD
diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index 4b23e4d..5b3fd53 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -1691,7 +1691,7 @@ int udhcpc_main(int argc UNUSED_PARAM, char **argv)
  * They say ISC DHCP client supports this case.
  */
 				server_addr = 0;
-				temp = udhcp_get_option(&packet, DHCP_SERVER_ID);
+				temp = udhcp_get_option32(&packet, DHCP_SERVER_ID);
 				if (!temp) {
 					bb_error_msg("no server ID, using 0.0.0.0");
 				} else {
@@ -1718,7 +1718,7 @@ int udhcpc_main(int argc UNUSED_PARAM, char **argv)
 				struct in_addr temp_addr;
 				uint8_t *temp;
 
-				temp = udhcp_get_option(&packet, DHCP_LEASE_TIME);
+				temp = udhcp_get_option32(&packet, DHCP_LEASE_TIME);
 				if (!temp) {
 					bb_error_msg("no lease time with ACK, using 1 hour lease");
 					lease_seconds = 60 * 60;
@@ -1813,7 +1813,7 @@ int udhcpc_main(int argc UNUSED_PARAM, char **argv)
 					uint32_t svid;
 					uint8_t *temp;
 
-					temp = udhcp_get_option(&packet, DHCP_SERVER_ID);
+					temp = udhcp_get_option32(&packet, DHCP_SERVER_ID);
 					if (!temp) {
  non_matching_svid:
 						log1("received DHCP NAK with wrong"
diff --git a/networking/udhcp/dhcpd.c b/networking/udhcp/dhcpd.c
index a8cd3f0..477856d 100644
--- a/networking/udhcp/dhcpd.c
+++ b/networking/udhcp/dhcpd.c
@@ -640,7 +640,7 @@ static void add_server_options(struct dhcp_packet *packet)
 static uint32_t select_lease_time(struct dhcp_packet *packet)
 {
 	uint32_t lease_time_sec = server_config.max_lease_sec;
-	uint8_t *lease_time_opt = udhcp_get_option(packet, DHCP_LEASE_TIME);
+	uint8_t *lease_time_opt = udhcp_get_option32(packet, DHCP_LEASE_TIME);
 	if (lease_time_opt) {
 		move_from_unaligned32(lease_time_sec, lease_time_opt);
 		lease_time_sec = ntohl(lease_time_sec);
@@ -987,7 +987,7 @@ int udhcpd_main(int argc UNUSED_PARAM, char **argv)
 		}
 
 		/* Get SERVER_ID if present */
-		server_id_opt = udhcp_get_option(&packet, DHCP_SERVER_ID);
+		server_id_opt = udhcp_get_option32(&packet, DHCP_SERVER_ID);
 		if (server_id_opt) {
 			uint32_t server_id_network_order;
 			move_from_unaligned32(server_id_network_order, server_id_opt);
@@ -1011,7 +1011,7 @@ int udhcpd_main(int argc UNUSED_PARAM, char **argv)
 		}
 
 		/* Get REQUESTED_IP if present */
-		requested_ip_opt = udhcp_get_option(&packet, DHCP_REQUESTED_IP);
+		requested_ip_opt = udhcp_get_option32(&packet, DHCP_REQUESTED_IP);
 		if (requested_ip_opt) {
 			move_from_unaligned32(requested_nip, requested_ip_opt);
 		}