netdev - [PATCH ipsec/vti 1/2] vti: fragment IPv4 packets when DF bit is not set

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Date:   Sun, 17 Mar 2019 23:37:56 +0000
From:   Bram Yvahk <bram-yvahk@...l.wizbit.be>
To:     steffen.klassert@...unet.com, herbert@...dor.apana.org.au,
        davem@...emloft.net
Cc:     netdev@...r.kernel.org
Subject: [PATCH ipsec/vti 1/2] vti: fragment IPv4 packets when DF bit is not set

Only send a 'need to frag' ICMP message when the "Don't Fragment" bit
is set. If it's not set then the packet can/will be fragmented.

This fixes sending an 'need to frag' message on a client that did not
set the DF bit, i.e.:

	$ ping -s 1300 -M dont -c5 192.168.235.2
	PING 192.168.235.3 (192.168.235.3) 1300(1328) bytes of data.
	From 192.168.236.254 icmp_seq=1 Frag needed and DF set (mtu = 1214)

Signed-off-by: Bram Yvahk <bram-yvahk@...l.wizbit.be>
---
 net/ipv4/ip_vti.c  | 43 +++++++++++++++++++++++++++--------------
 net/ipv6/ip6_vti.c | 56 +++++++++++++++++++++++++++++++++++++++---------------
 2 files changed, 70 insertions(+), 29 deletions(-)

diff --git a/net/ipv4/ip_vti.c b/net/ipv4/ip_vti.c
index 68a21bf..5738e44 100644
--- a/net/ipv4/ip_vti.c
+++ b/net/ipv4/ip_vti.c
@@ -196,6 +196,34 @@ static bool vti_state_check(const struct xfrm_state *x, __be32 dst, __be32 src)
 	return true;
 }
 
+static bool vti_tunnel_check_size(struct sk_buff *skb)
+{
+	int mtu;
+
+	if (skb->protocol == htons(ETH_P_IP)) {
+		if (!(ip_hdr(skb)->frag_off & htons(IP_DF)) || skb->ignore_df)
+			return true;
+	}
+
+	mtu = dst_mtu(skb_dst(skb));
+	if (skb->len > mtu) {
+		skb_dst_update_pmtu(skb, mtu);
+		if (skb->protocol == htons(ETH_P_IP)) {
+			icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
+				  htonl(mtu));
+		} else {
+			if (mtu < IPV6_MIN_MTU)
+				mtu = IPV6_MIN_MTU;
+
+			icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
+		}
+
+		return false;
+	}
+
+	return true;
+}
+
 static netdev_tx_t vti_xmit(struct sk_buff *skb, struct net_device *dev,
 			    struct flowi *fl)
 {
@@ -205,7 +233,6 @@ static netdev_tx_t vti_xmit(struct sk_buff *skb, struct net_device *dev,
 	struct net_device *tdev;	/* Device to other host */
 	int pkt_len = skb->len;
 	int err;
-	int mtu;
 
 	if (!dst) {
 		dev->stats.tx_carrier_errors++;
@@ -233,19 +260,7 @@ static netdev_tx_t vti_xmit(struct sk_buff *skb, struct net_device *dev,
 		goto tx_error;
 	}
 
-	mtu = dst_mtu(dst);
-	if (skb->len > mtu) {
-		skb_dst_update_pmtu(skb, mtu);
-		if (skb->protocol == htons(ETH_P_IP)) {
-			icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
-				  htonl(mtu));
-		} else {
-			if (mtu < IPV6_MIN_MTU)
-				mtu = IPV6_MIN_MTU;
-
-			icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
-		}
-
+	if (!vti_tunnel_check_size(skb)) {
 		dst_release(dst);
 		goto tx_error;
 	}
diff --git a/net/ipv6/ip6_vti.c b/net/ipv6/ip6_vti.c
index 8b6eeff..47f178c 100644
--- a/net/ipv6/ip6_vti.c
+++ b/net/ipv6/ip6_vti.c
@@ -436,6 +436,46 @@ static bool vti6_state_check(const struct xfrm_state *x,
 }
 
 /**
+ * vti6_tunnel_check_size - check size of packet
+ *   @skb: the outgoing socket buffer
+ *
+ * Description:
+ *   Check if packet is too large (> pmtu)
+ *
+ * Return:
+ *   true if size of packet is ok
+ *   false if packet is too large
+ **/
+static bool vti6_tunnel_check_size(struct sk_buff *skb)
+{
+	int mtu;
+
+	if (skb->protocol == htons(ETH_P_IP)) {
+		if (!(ip_hdr(skb)->frag_off & htons(IP_DF)) || skb->ignore_df)
+			return true;
+	}
+
+	mtu = dst_mtu(skb_dst(skb));
+	if (skb->len > mtu) {
+		skb_dst_update_pmtu(skb, mtu);
+
+		if (skb->protocol == htons(ETH_P_IPV6)) {
+			if (mtu < IPV6_MIN_MTU)
+				mtu = IPV6_MIN_MTU;
+
+			icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
+		} else {
+			icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
+				  htonl(mtu));
+		}
+
+		return false;
+	}
+
+	return true;
+}
+
+/**
  * vti6_xmit - send a packet
  *   @skb: the outgoing socket buffer
  *   @dev: the outgoing tunnel device
@@ -451,7 +491,6 @@ vti6_xmit(struct sk_buff *skb, struct net_device *dev, struct flowi *fl)
 	struct xfrm_state *x;
 	int pkt_len = skb->len;
 	int err = -1;
-	int mtu;
 
 	if (!dst)
 		goto tx_err_link_failure;
@@ -481,20 +520,7 @@ vti6_xmit(struct sk_buff *skb, struct net_device *dev, struct flowi *fl)
 		goto tx_err_dst_release;
 	}
 
-	mtu = dst_mtu(dst);
-	if (skb->len > mtu) {
-		skb_dst_update_pmtu(skb, mtu);
-
-		if (skb->protocol == htons(ETH_P_IPV6)) {
-			if (mtu < IPV6_MIN_MTU)
-				mtu = IPV6_MIN_MTU;
-
-			icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
-		} else {
-			icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
-				  htonl(mtu));
-		}
-
+	if (!vti6_tunnel_check_size(skb)) {
 		err = -EMSGSIZE;
 		goto tx_err_dst_release;
 	}
-- 
2.7.0