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[PATCH] net: dev_forward_skb(): Scrub packet's per-netns info only when crossing netns

Before this commit, dev_forward_skb() always cleared packet's
per-network-namespace info. Even if the packet doesn't cross
network namespaces.

The comment above dev_forward_skb() describes that this is done
because the receiving device may be in another network namespace.
However, this case can easily be tested for and therefore we can
scrub packet's per-network-namespace info only when receiving device
is indeed in another network namespace.

Therefore, this commit changes ____dev_forward_skb() to tell
skb_scrub_packet() that skb has crossed network-namespace only in case
transmitting device (skb->dev) network namespace is different then
receiving device (dev) network namespace.

An example of a netdev that use skb_forward_skb() is veth.
Thus, before this commit a packet transmitted from one veth peer to
another when both veth peers are on same network namespace will lose
it's skb->mark. The bug could easily be demonstrated by the following:

ip netns add test
ip netns exec test bash
ip link add veth-a type veth peer name veth-b
ip link set veth-a up
ip link set veth-b up
ip addr add dev veth-a
tc qdisc add dev veth-a root handle 1 prio
tc qdisc add dev veth-b ingress
tc filter add dev veth-a parent 1: u32 match u32 0 0 action skbedit mark 1337
tc filter add dev veth-b parent ffff: basic match 'meta(nf_mark eq 1337)' action simple "skb->mark 1337!"
dmesg -C

Before this change, the above will print nothing to dmesg.
After this change, "skb->mark 1337!" will be printed as necessary.

Signed-off-by: Liran Alon <liran.alon@xxxxxxxxxx>
Reviewed-by: Yuval Shaia <yuval.shaia@xxxxxxxxxx>
Signed-off-by: Yuval Shaia <yuval.shaia@xxxxxxxxxx>
 include/linux/netdevice.h | 2 +-
 net/core/dev.c            | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 5eef6c8e2741..5908f1e31ee2 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3371,7 +3371,7 @@ static __always_inline int ____dev_forward_skb(struct net_device *dev,
 		return NET_RX_DROP;
-	skb_scrub_packet(skb, true);
+	skb_scrub_packet(skb, !net_eq(dev_net(dev), dev_net(skb->dev)));
 	skb->priority = 0;
 	return 0;
diff --git a/net/core/dev.c b/net/core/dev.c
index 2cedf520cb28..087787dd0a50 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1877,9 +1877,9 @@ int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
  * start_xmit function of one device into the receive queue
  * of another device.
- * The receiving device may be in another namespace, so
- * we have to clear all information in the skb that could
- * impact namespace isolation.
+ * The receiving device may be in another namespace.
+ * In that case, we have to clear all information in the
+ * skb that could impact namespace isolation.
 int dev_forward_skb(struct net_device *dev, struct sk_buff *skb)