macsec: fix NULL dereference in macsec_upd_offload()

macsec_upd_offload() gets the value of MACSEC_OFFLOAD_ATTR_TYPE
without checking its presence in the request message, and this causes
a NULL dereference. Fix it rejecting any configuration that does not
include this attribute.

Reported-and-tested-by: syzbot+7022ab7c383875c17eff@syzkaller.appspotmail.com
Fixes: dcb780f ("net: macsec: add nla support for changing the offloading selection")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macsec.c

@@ -2594,6 +2594,9 @@ static int macsec_upd_offload(struct sk_buff *skb, struct genl_info *info)
 		return PTR_ERR(dev);
 	macsec = macsec_priv(dev);
 
+	if (!tb_offload[MACSEC_OFFLOAD_ATTR_TYPE])
+		return -EINVAL;
+
 	offload = nla_get_u8(tb_offload[MACSEC_OFFLOAD_ATTR_TYPE]);
 	if (macsec->offload == offload)
 		return 0;