tipc: check link name with right length in tipc_nl_compat_link_set

A similar issue as fixed by Patch "tipc: check bearer name with right
length in tipc_nl_compat_bearer_enable" was also found by syzbot in
tipc_nl_compat_link_set().

The length to check with should be 'TLV_GET_DATA_LEN(msg->req) -
offsetof(struct tipc_link_config, name)'.

Reported-by: syzbot+de00a87b8644a582ae79@syzkaller.appspotmail.com
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/netlink_compat.c

@@ -771,7 +771,12 @@ static int tipc_nl_compat_link_set(struct tipc_nl_compat_cmd_doit *cmd,
 
 	lc = (struct tipc_link_config *)TLV_DATA(msg->req);
 
-	len = min_t(int, TLV_GET_DATA_LEN(msg->req), TIPC_MAX_LINK_NAME);
+	len = TLV_GET_DATA_LEN(msg->req);
+	len -= offsetof(struct tipc_link_config, name);
+	if (len <= 0)
+		return -EINVAL;
+
+	len = min_t(int, len, TIPC_MAX_LINK_NAME);
 	if (!string_is_valid(lc->name, len))
 		return -EINVAL;