dhcp-network: add check for DHCP.chaddr

Check that received DHCP packets actually include our MAC address in
chaddr field. BPF interpreter has 32 bit wide registers but MAC address
is 48 bits long so we have to do check in two steps.

4 files changed, 18 insertions, 6 deletions

diff --git a/src/libsystemd-network/dhcp-internal.h b/src/libsystemd-network/dhcp-internal.h
index 6f6f1218d9..1069c8a03b 100644
--- a/src/libsystemd-network/dhcp-internal.h
+++ b/src/libsystemd-network/dhcp-internal.h
@@ -24,13 +24,14 @@
 
 #include <stdint.h>
 #include <linux/if_packet.h>
+#include <net/ethernet.h>
 
 #include "socket-util.h"
 
 #include "sd-dhcp-client.h"
 #include "dhcp-protocol.h"
 
-int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link, uint32_t xid);
+int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link, uint32_t xid, struct ether_addr mac_addr);
 int dhcp_network_bind_udp_socket(be32_t address, uint16_t port);
 int dhcp_network_send_raw_socket(int s, const union sockaddr_union *link,
                                  const void *packet, size_t len);
diff --git a/src/libsystemd-network/dhcp-network.c b/src/libsystemd-network/dhcp-network.c
index 9d579177a4..f119cae7ff 100644
--- a/src/libsystemd-network/dhcp-network.c
+++ b/src/libsystemd-network/dhcp-network.c
@@ -33,7 +33,7 @@
 #include "dhcp-internal.h"
 
 int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link,
-                                 uint32_t xid) {
+                                 uint32_t xid, struct ether_addr mac_addr) {
         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 ? */
@@ -60,7 +60,18 @@ int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link,
             BPF_STMT(BPF_LD + BPF_W + BPF_ABS, offsetof(DHCPPacket, dhcp.xid)),    /* A <- client identifier */
             BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, xid, 1, 0),                        /* client identifier == xid ? */
             BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
-            /* TODO: match chaddr */
+            BPF_STMT(BPF_LD + BPF_IMM, htobe32(*((unsigned int *) &mac_addr))),                    /* A <- 4 bytes of client's MAC */
+            BPF_STMT(BPF_MISC + BPF_TAX, 0),                                                       /* X <- A */
+            BPF_STMT(BPF_LD + BPF_W + BPF_ABS, offsetof(DHCPPacket, dhcp.chaddr)),                 /* A <- 4 bytes of MAC from dhcp.chaddr */
+            BPF_STMT(BPF_ALU + BPF_XOR + BPF_X, 0),                                                /* A xor X */
+            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 1, 0),                                          /* A == 0 ? */
+            BPF_STMT(BPF_RET + BPF_K, 0),                                                          /* ignore */
+            BPF_STMT(BPF_LD + BPF_IMM, htobe16(*((unsigned short *) (((char *) &mac_addr) + 4)))), /* A <- remainder of client's MAC */
+            BPF_STMT(BPF_MISC + BPF_TAX, 0),                                                       /* X <- A */
+            BPF_STMT(BPF_LD + BPF_H + BPF_ABS, offsetof(DHCPPacket, dhcp.chaddr) + 4),             /* A <- remainder of MAC from dhcp.chaddr */
+            BPF_STMT(BPF_ALU + BPF_XOR + BPF_X, 0),                                                /* A xor X */
+            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0, 1, 0),                                          /* A == 0 ? */
+            BPF_STMT(BPF_RET + BPF_K, 0),                                                          /* ignore */
             BPF_STMT(BPF_LD + BPF_W + BPF_ABS, offsetof(DHCPPacket, dhcp.magic)),  /* A <- DHCP magic cookie */
             BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, DHCP_MAGIC_COOKIE, 1, 0),          /* cookie == DHCP magic cookie ? */
             BPF_STMT(BPF_RET + BPF_K, 0),                                          /* ignore */
diff --git a/src/libsystemd-network/sd-dhcp-client.c b/src/libsystemd-network/sd-dhcp-client.c
index d8a9d20e4c..6b19666c3b 100644
--- a/src/libsystemd-network/sd-dhcp-client.c
+++ b/src/libsystemd-network/sd-dhcp-client.c
@@ -751,7 +751,7 @@ static int client_start(sd_dhcp_client *client) {
 
         client->xid = random_u32();
 
-        r = dhcp_network_bind_raw_socket(client->index, &client->link, client->xid);
+        r = dhcp_network_bind_raw_socket(client->index, &client->link, client->xid, client->client_id.mac_addr);
         if (r < 0) {
                 client_stop(client, r);
                 return r;
@@ -795,7 +795,7 @@ static int client_timeout_t2(sd_event_source *s, uint64_t usec, void *userdata)
         client->state = DHCP_STATE_REBINDING;
         client->attempt = 1;
 
-        r = dhcp_network_bind_raw_socket(client->index, &client->link, client->xid);
+        r = dhcp_network_bind_raw_socket(client->index, &client->link, client->xid, client->client_id.mac_addr);
         if (r < 0) {
                 client_stop(client, r);
                 return 0;
diff --git a/src/libsystemd-network/test-dhcp-client.c b/src/libsystemd-network/test-dhcp-client.c
index 450b6d4d3c..7cbe10d5a7 100644
--- a/src/libsystemd-network/test-dhcp-client.c
+++ b/src/libsystemd-network/test-dhcp-client.c
@@ -196,7 +196,7 @@ int dhcp_network_send_raw_socket(int s, const union sockaddr_union *link,
         return 575;
 }
 
-int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link, uint32_t id)
+int dhcp_network_bind_raw_socket(int index, union sockaddr_union *link, uint32_t id, struct ether_addr mac)
 {
         if (socketpair(AF_UNIX, SOCK_STREAM, 0, test_fd) < 0)
                 return -errno;