hsr: Prevent use after free in prp_create_tagged_frame()

The prp_fill_rct() function can fail.  In that situation, it frees the
skb and returns NULL.  Meanwhile on the success path, it returns the
original skb.  So it's straight forward to fix bug by using the returned
value.

Fixes: 451d812 ("net: prp: add packet handling support")
Signed-off-by: Dan Carpenter <[email protected]>
Acked-by: Paolo Abeni <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Dan Carpenter

net/hsr/hsr_forward.c

@@ -342,9 +342,7 @@ struct sk_buff *prp_create_tagged_frame(struct hsr_frame_info *frame,
 	skb = skb_copy_expand(frame->skb_std, 0,
 			      skb_tailroom(frame->skb_std) + HSR_HLEN,
 			      GFP_ATOMIC);
-	prp_fill_rct(skb, frame, port);
-
-	return skb;
+	return prp_fill_rct(skb, frame, port);
 }
 
 static void hsr_deliver_master(struct sk_buff *skb, struct net_device *dev,