net: hsr: check skb can contain struct hsr_ethhdr in fill_frame_info

Check at start of fill_frame_info that the MAC header in the supplied
skb is large enough to fit a struct hsr_ethhdr, as otherwise this is
not a valid HSR frame. If it is too small, return an error which will
then cause the callers to clean up the skb. Fixes a KMSAN-found
uninit-value bug reported by syzbot at:
https://syzkaller.appspot.com/bug?id=f7e9b601f1414f814f7602a82b6619a8d80bce3f

Reported-by: syzbot+e267bed19bfc5478fb33@syzkaller.appspotmail.com
Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/hsr/hsr_forward.c

@@ -520,6 +520,10 @@ static int fill_frame_info(struct hsr_frame_info *frame,
 	struct ethhdr *ethhdr;
 	__be16 proto;
 
+	/* Check if skb contains hsr_ethhdr */
+	if (skb->mac_len < sizeof(struct hsr_ethhdr))
+		return -EINVAL;
+
 	memset(frame, 0, sizeof(*frame));
 	frame->is_supervision = is_supervision_frame(port->hsr, skb);
 	frame->node_src = hsr_get_node(port, &hsr->node_db, skb,