dev: introduce skb_scrub_packet()

The goal of this new function is to perform all needed cleanup before sending
an skb into another netns.
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -2384,6 +2384,7 @@ extern void	       skb_split(struct sk_buff *skb,
 				 struct sk_buff *skb1, const u32 len);
 extern int	       skb_shift(struct sk_buff *tgt, struct sk_buff *skb,
 				 int shiftlen);
+extern void	       skb_scrub_packet(struct sk_buff *skb);
 
 extern struct sk_buff *skb_segment(struct sk_buff *skb,
 				   netdev_features_t features);

net/core/dev.c

@@ -1652,22 +1652,13 @@ int dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
 		}
 	}
 
-	skb_orphan(skb);
-
 	if (unlikely(!is_skb_forwardable(dev, skb))) {
 		atomic_long_inc(&dev->rx_dropped);
 		kfree_skb(skb);
 		return NET_RX_DROP;
 	}
-	skb->skb_iif = 0;
-	skb_dst_drop(skb);
-	skb->tstamp.tv64 = 0;
-	skb->pkt_type = PACKET_HOST;
+	skb_scrub_packet(skb);
 	skb->protocol = eth_type_trans(skb, dev);
-	skb->mark = 0;
-	secpath_reset(skb);
-	nf_reset(skb);
-	nf_reset_trace(skb);
 	return netif_rx(skb);
 }
 EXPORT_SYMBOL_GPL(dev_forward_skb);

net/core/skbuff.c

@@ -3492,3 +3492,26 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
 	return true;
 }
 EXPORT_SYMBOL(skb_try_coalesce);
+
+/**
+ * skb_scrub_packet - scrub an skb before sending it to another netns
+ *
+ * @skb: buffer to clean
+ *
+ * skb_scrub_packet can be used to clean an skb before injecting it in
+ * another namespace. We have to clear all information in the skb that
+ * could impact namespace isolation.
+ */
+void skb_scrub_packet(struct sk_buff *skb)
+{
+	skb_orphan(skb);
+	skb->tstamp.tv64 = 0;
+	skb->pkt_type = PACKET_HOST;
+	skb->skb_iif = 0;
+	skb_dst_drop(skb);
+	skb->mark = 0;
+	secpath_reset(skb);
+	nf_reset(skb);
+	nf_reset_trace(skb);
+}
+EXPORT_SYMBOL_GPL(skb_scrub_packet);