[systemd-devel] [PATCH 1/2] dhcp-network: add check for DHCP.chaddr

[Michal Sekletar]

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.
---
 src/libsystemd-network/dhcp-internal.h    |  3 ++-
 src/libsystemd-network/dhcp-network.c     | 15 +++++++++++++--
 src/libsystemd-network/sd-dhcp-client.c   |  4 ++--
 src/libsystemd-network/test-dhcp-client.c |  2 +-
 4 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/src/libsystemd-network/dhcp-internal.h b/src/libsystemd-network/dhcp-internal.h
index 03cc824..fd86a70 100644
--- a/src/libsystemd-network/dhcp-internal.h
+++ b/src/libsystemd-network/dhcp-internal.h
@@ -24,12 +24,13 @@
 
 #include <stdint.h>
 #include <linux/if_packet.h>
+#include <net/ethernet.h>
 
 #include "socket-util.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 d7ba0ef..e5fd6a8 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 ? */
@@ -56,7 +56,18 @@ 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 c1af6df..d44cad2 100644
--- a/src/libsystemd-network/sd-dhcp-client.c
+++ b/src/libsystemd-network/sd-dhcp-client.c
@@ -719,7 +719,7 @@ 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;
@@ -761,7 +761,7 @@ 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 450b6d4..7cbe10d 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;
-- 
2.0.0