net: Add a xfrm validate function to validate_xmit_skb

When we do IPsec offloading, we need a fallback for
packets that were targeted to be IPsec offloaded but
rerouted to a device that does not support IPsec offload.
For that we add a function that checks the offloading
features of the sending device and and flags the
requirement of a fallback before it calls the IPsec
output function. The IPsec output function adds the IPsec
trailer and does encryption if needed.
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/xfrm.h

@@ -1862,6 +1862,7 @@ static inline struct xfrm_offload *xfrm_offload(struct sk_buff *skb)
 
 #ifdef CONFIG_XFRM_OFFLOAD
 void __net_init xfrm_dev_init(void);
+int validate_xmit_xfrm(struct sk_buff *skb, netdev_features_t features);
 int xfrm_dev_state_add(struct net *net, struct xfrm_state *x,
 		       struct xfrm_user_offload *xuo);
 bool xfrm_dev_offload_ok(struct sk_buff *skb, struct xfrm_state *x);
@@ -1890,6 +1891,11 @@ static inline void __net_init xfrm_dev_init(void)
 {
 }
 
+static inline int validate_xmit_xfrm(struct sk_buff *skb, netdev_features_t features)
+{
+	return 0;
+}
+
 static inline int xfrm_dev_state_add(struct net *net, struct xfrm_state *x, struct xfrm_user_offload *xuo)
 {
 	return 0;

net/core/dev.c

@@ -2972,6 +2972,9 @@ static struct sk_buff *validate_xmit_skb(struct sk_buff *skb, struct net_device
 		    __skb_linearize(skb))
 			goto out_kfree_skb;
 
+		if (validate_xmit_xfrm(skb, features))
+			goto out_kfree_skb;
+
 		/* If packet is not checksummed and device does not
 		 * support checksumming for this protocol, complete
 		 * checksumming here.

net/xfrm/xfrm_device.c

@@ -22,6 +22,35 @@
 #include <net/xfrm.h>
 #include <linux/notifier.h>
 
+int validate_xmit_xfrm(struct sk_buff *skb, netdev_features_t features)
+{
+	int err;
+	struct xfrm_state *x;
+	struct xfrm_offload *xo = xfrm_offload(skb);
+
+	if (skb_is_gso(skb))
+		return 0;
+
+	if (xo) {
+		x = skb->sp->xvec[skb->sp->len - 1];
+		if (xo->flags & XFRM_GRO || x->xso.flags & XFRM_OFFLOAD_INBOUND)
+			return 0;
+
+		x->outer_mode->xmit(x, skb);
+
+		err = x->type_offload->xmit(x, skb, features);
+		if (err) {
+			XFRM_INC_STATS(xs_net(x), LINUX_MIB_XFRMOUTSTATEPROTOERROR);
+			return err;
+		}
+
+		skb_push(skb, skb->data - skb_mac_header(skb));
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(validate_xmit_xfrm);
+
 int xfrm_dev_state_add(struct net *net, struct xfrm_state *x,
 		       struct xfrm_user_offload *xuo)
 {