[PATCH v3 net 5/9] wireguard: device: use icmp_ndo_send helper and remove skb_share_check

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Because wireguard is calling icmp from network device context, it should
use the ndo helper so that the rate limiting applies correctly. Also,
skb_share_check in the xmit path is an impossible condition to reach; an
skb in ndo_start_xmit won't be shared by definition.

This commit adds a small test to the wireguard test suite to ensure that
the new functions continue doing the right thing in the context of
wireguard. It does this by setting up a condition that will definately
evoke an icmp error message from the driver, but along a nat'd path.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://lore.kernel.org/netdev/CAHmME9pk8HEFRq_mBeatNbwXTx7UEfiQ_HG_+Lyz7E+80GmbSA@mail.gmail.com/
---
 drivers/net/wireguard/device.c             |  8 ++------
 tools/testing/selftests/wireguard/netns.sh | 10 ++++++++++
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/drivers/net/wireguard/device.c b/drivers/net/wireguard/device.c
index 16b19824b9ad..62cb7b106c52 100644
--- a/drivers/net/wireguard/device.c
+++ b/drivers/net/wireguard/device.c
@@ -167,10 +167,6 @@ static netdev_tx_t wg_xmit(struct sk_buff *skb, struct net_device *dev)
 	skb_list_walk_safe(skb, skb, next) {
 		skb_mark_not_on_list(skb);
 
-		skb = skb_share_check(skb, GFP_ATOMIC);
-		if (unlikely(!skb))
-			continue;
-
 		/* We only need to keep the original dst around for icmp,
 		 * so at this point we're in a position to drop it.
 		 */
@@ -203,9 +199,9 @@ static netdev_tx_t wg_xmit(struct sk_buff *skb, struct net_device *dev)
 err:
 	++dev->stats.tx_errors;
 	if (skb->protocol == htons(ETH_P_IP))
-		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_HOST_UNREACH, 0);
+		icmp_ndo_send(skb, ICMP_DEST_UNREACH, ICMP_HOST_UNREACH, 0);
 	else if (skb->protocol == htons(ETH_P_IPV6))
-		icmpv6_send(skb, ICMPV6_DEST_UNREACH, ICMPV6_ADDR_UNREACH, 0);
+		icmpv6_ndo_send(skb, ICMPV6_DEST_UNREACH, ICMPV6_ADDR_UNREACH, 0);
 	kfree_skb(skb);
 	return ret;
 }
diff --git a/tools/testing/selftests/wireguard/netns.sh b/tools/testing/selftests/wireguard/netns.sh
index f5ab1cda8bb5..4e31d5b1bf7f 100755
--- a/tools/testing/selftests/wireguard/netns.sh
+++ b/tools/testing/selftests/wireguard/netns.sh
@@ -297,7 +297,17 @@ ip1 -4 rule add table main suppress_prefixlength 0
 n1 ping -W 1 -c 100 -f 192.168.99.7
 n1 ping -W 1 -c 100 -f abab::1111
 
+# Have ns2 NAT into wg0 packets from ns0, but return an icmp error along the right route.
+n2 iptables -t nat -A POSTROUTING -s 10.0.0.0/24 -d 192.168.241.0/24 -j SNAT --to 192.168.241.2
+n0 iptables -t filter -A INPUT \! -s 10.0.0.0/24 -i vethrs -j DROP # Manual rpfilter just to be explicit.
+n2 bash -c 'printf 1 > /proc/sys/net/ipv4/ip_forward'
+ip0 -4 route add 192.168.241.1 via 10.0.0.100
+n2 wg set wg0 peer "$pub1" remove
+[[ $(! n0 ping -W 1 -c 1 192.168.241.1 || false) == *"From 10.0.0.100 icmp_seq=1 Destination Host Unreachable"* ]]
+
 n0 iptables -t nat -F
+n0 iptables -t filter -F
+n2 iptables -t nat -F
 ip0 link del vethrc
 ip0 link del vethrs
 ip1 link del wg0
-- 
2.25.0