sd-dhcp-client: eagerly drop too small packets

If they are too small to fit the IP+UDP+DHCP headers they can be of no use, so
don't waste resources parsing them. This is at the cost of losing some verbosity
in the logging.

3 files changed, 12 insertions, 24 deletions

diff --git a/src/libsystemd-network/dhcp-network.c b/src/libsystemd-network/dhcp-network.c
index 8bfb2d50af..a9a15b4d5a 100644
--- a/src/libsystemd-network/dhcp-network.c
+++ b/src/libsystemd-network/dhcp-network.c
@@ -34,11 +34,14 @@
 int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link)
 {
         struct sock_filter filter[] = {
+            BPF_STMT(BPF_LD + BPF_W + BPF_LEN, 0),                                 /* A <- packet length */
+            BPF_JUMP(BPF_JMP + BPF_JGE + BPF_K, sizeof(DHCPPacket), 1, 0),         /* packet >= DHCPPacket ? */
+            BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
             BPF_STMT(BPF_LD + BPF_B + BPF_ABS, offsetof(DHCPPacket, ip.protocol)), /* A <- IP protocol */
-            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 1, 0),                /* IP protocol = UDP? */
+            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 1, 0),                /* IP protocol == UDP ? */
             BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
             BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(DHCPPacket, udp.dest)),    /* A <- UDP destination port */
-            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, DHCP_PORT_CLIENT, 1, 0),           /* UDP destination port = DHCP client? */
+            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, DHCP_PORT_CLIENT, 1, 0),           /* UDP destination port == DHCP client port ? */
             BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
             BPF_STMT(BPF_RET + BPF_K, 65535),                                      /* return all */
         };
diff --git a/src/libsystemd-network/dhcp-packet.c b/src/libsystemd-network/dhcp-packet.c
index 0549f575ac..102ed096af 100644
--- a/src/libsystemd-network/dhcp-packet.c
+++ b/src/libsystemd-network/dhcp-packet.c
@@ -113,13 +113,6 @@ int dhcp_packet_verify_headers(DHCPPacket *packet, size_t len, bool checksum) {
 
         /* IP */
 
-        if (len < DHCP_IP_SIZE) {
-                log_dhcp_client(client, "ignoring packet: packet (%zu bytes) "
-                                " smaller than IP header (%u bytes)", len,
-                                DHCP_IP_SIZE);
-                return -EINVAL;
-        }
-
         if (packet->ip.version != IPVERSION) {
                 log_dhcp_client(client, "ignoring packet: not IPv4");
                 return -EINVAL;
@@ -152,13 +145,6 @@ int dhcp_packet_verify_headers(DHCPPacket *packet, size_t len, bool checksum) {
                 return -EINVAL;
         }
 
-        if (len < DHCP_IP_UDP_SIZE) {
-                log_dhcp_client(client, "ignoring packet: packet (%zu bytes) "
-                                " smaller than IP+UDP header (%u bytes)", len,
-                                DHCP_IP_UDP_SIZE);
-                return -EINVAL;
-        }
-
         if (len < hdrlen + be16toh(packet->udp.len)) {
                 log_dhcp_client(client, "ignoring packet: packet (%zu bytes) "
                                 "smaller than expected (%zu) by UDP header", len,
diff --git a/src/libsystemd-network/sd-dhcp-client.c b/src/libsystemd-network/sd-dhcp-client.c
index 5824e6ee91..722f862832 100644
--- a/src/libsystemd-network/sd-dhcp-client.c
+++ b/src/libsystemd-network/sd-dhcp-client.c
@@ -931,12 +931,6 @@ static int client_handle_message(sd_dhcp_client *client, DHCPMessage *message,
         assert(client->event);
         assert(message);
 
-        if (len < DHCP_MESSAGE_SIZE) {
-                log_dhcp_client(client, "message too small (%d bytes): "
-                                "ignoring", len);
-                return 0;
-        }
-
         if (be32toh(message->magic) != DHCP_MAGIC_COOKIE) {
                 log_dhcp_client(client, "not a DHCP message: ignoring");
                 return 0;
@@ -1081,7 +1075,11 @@ static int client_receive_message_udp(sd_event_source *s, int fd,
                 return -ENOMEM;
 
         len = read(fd, message, buflen);
-        if (len < 0)
+        if (len < 0) {
+                log_dhcp_client(client, "could not receive message from UDP "
+                                "socket: %s", strerror(errno));
+                return 0;
+        } else if ((size_t)len < sizeof(DHCPMessage))
                 return 0;
 
         return client_handle_message(client, message, len);
@@ -1122,7 +1120,8 @@ static int client_receive_message_raw(sd_event_source *s, int fd,
                 log_dhcp_client(client, "could not receive message from raw "
                                 "socket: %s", strerror(errno));
                 return 0;
-        }
+        } else if ((size_t)len < sizeof(DHCPPacket))
+                return 0;
 
         for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
                 if (cmsg->cmsg_level == SOL_PACKET && cmsg->cmsg_type == PACKET_AUXDATA) {