tipc: fix null-ptr-deref when acquiring remote ip of ethernet bearer

The reproduction steps:
1. create a tun interface
2. enable l2 bearer
3. TIPC_NL_UDP_GET_REMOTEIP with media name set to tun

tipc: Started in network mode
tipc: Node identity 8af312d38a21, cluster identity 4711
tipc: Enabled bearer <eth:syz_tun>, priority 1
Oops: general protection fault
KASAN: null-ptr-deref in range
CPU: 1 UID: 1000 PID: 559 Comm: poc Not tainted 6.16.0-rc1+ #117 PREEMPT
Hardware name: QEMU Ubuntu 24.04 PC
RIP: 0010:tipc_udp_nl_dump_remoteip+0x4a4/0x8f0

the ub was in fact a struct dev.

when bid != 0 && skip_cnt != 0, bearer_list[bid] may be NULL or
other media when other thread changes it.

fix this by checking media_id.

Fixes: 832629c ("tipc: add UDP remoteip dump to netlink API")
Signed-off-by: Haixia Qu <[email protected]>
Reviewed-by: Tung Nguyen <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Haixia Qu

net/tipc/udp_media.c

@@ -489,7 +489,7 @@ int tipc_udp_nl_dump_remoteip(struct sk_buff *skb, struct netlink_callback *cb)
 
 		rtnl_lock();
 		b = tipc_bearer_find(net, bname);
-		if (!b) {
+		if (!b || b->bcast_addr.media_id != TIPC_MEDIA_TYPE_UDP) {
 			rtnl_unlock();
 			return -EINVAL;
 		}
@@ -500,7 +500,7 @@ int tipc_udp_nl_dump_remoteip(struct sk_buff *skb, struct netlink_callback *cb)
 
 		rtnl_lock();
 		b = rtnl_dereference(tn->bearer_list[bid]);
-		if (!b) {
+		if (!b || b->bcast_addr.media_id != TIPC_MEDIA_TYPE_UDP) {
 			rtnl_unlock();
 			return -EINVAL;
 		}