Given:
- tap0 and vxlan0 are bridged
- vxlan0 stacked on eth0, eth0 having small mtu (e.g. 1400)
Assume GSO skbs arriving from tap0 having a gso_size as determined by
user-provided virtio_net_hdr (e.g. 1460 corresponding to VM mtu of 1500).
After encapsulation these skbs have skb_gso_network_seglen that exceed
eth0's ip_skb_dst_mtu.
These skbs are accidentally passed to ip_finish_output2 AS IS.
Alas, each final segment (segmented either by validate_xmit_skb or by
hardware UFO) would be larger than eth0 mtu.
As a result, those above-mtu segments get dropped on certain networks.
This behavior is not aligned with the NON-GSO case:
Assume a non-gso 1500-sized IP packet arrives from tap0. After
encapsulation, the vxlan datagram is fragmented normally at the
ip_finish_output-->ip_fragment code path.
The expected behavior for the GSO case would be segmenting the
"gso-oversized" skb first, then fragmenting each segment according to
dst mtu, and finally passing the resulting fragments to ip_finish_output2.
'ip_finish_output_gso' already supports this "Slowpath" behavior,
but it is only considered if IPSKB_FORWARDED is set (which is not set in
the bridged case).
In order to support the bridged case, we'll mark skbs arriving from an
ingress interface that get udp-encaspulated as "allowed to be fragmented".
This mark (as well as the original IPSKB_FORWARDED mark) gets tested in
'ip_finish_output_gso', in order to determine whether validating the
network seglen is needed.
Note the TUNNEL_DONT_FRAGMENT tun_flag is still honoured (both in the
gso and non-gso cases), which serves users wishing to forbid
fragmentation at the udp tunnel endpoint.
Cc: Hannes Frederic Sowa <han...@stressinduktion.org>
Cc: Florian Westphal <f...@strlen.de>
Signed-off-by: Shmulik Ladkani <shmulik.ladk...@gmail.com>
---
v2: Instead of completely removing the IPSKB_FORWARDED condition of
'ip_finish_output_gso' (forcing an expensive 'skb_gso_validate_mtu'
on all local traffic), augment the condition to the tunneled
usecase, as suggested by Florian and Hannes.
include/net/ip.h | 1 +
net/ipv4/ip_output.c | 10 +++++++---
net/ipv4/ip_tunnel_core.c | 9 +++++++++
3 files changed, 17 insertions(+), 3 deletions(-)
diff --git a/include/net/ip.h b/include/net/ip.h
index 08f36cd2b8..9742b92dc9 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -47,6 +47,7 @@ struct inet_skb_parm {
#define IPSKB_REROUTED BIT(4)
#define IPSKB_DOREDIRECT BIT(5)
#define IPSKB_FRAG_PMTU BIT(6)
+#define IPSKB_FRAG_SEGS BIT(7)
u16 frag_max_size;
};
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index e23f141c9b..18bb7639dd 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -221,11 +221,15 @@ static int ip_finish_output_gso(struct net *net, struct
sock *sk,
{
netdev_features_t features;
struct sk_buff *segs;
+ int allow_frag;
int ret = 0;
- /* common case: locally created skb or seglen is <= mtu */
- if (((IPCB(skb)->flags & IPSKB_FORWARDED) == 0) ||
- skb_gso_validate_mtu(skb, mtu))
+ allow_frag = IPCB(skb)->flags & (IPSKB_FORWARDED | IPSKB_FRAG_SEGS);
+
+ /* common case: locally created skb and fragmentation of segments is
+ * not allowed, or seglen is <= mtu
+ */
+ if (!allow_frag || skb_gso_validate_mtu(skb, mtu))
return ip_finish_output2(net, sk, skb);
/* Slowpath - GSO segment length is exceeding the dst MTU.
diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index afd6b5968c..9d847c3025 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -63,6 +63,7 @@ void iptunnel_xmit(struct sock *sk, struct rtable *rt, struct
sk_buff *skb,
int pkt_len = skb->len - skb_inner_network_offset(skb);
struct net *net = dev_net(rt->dst.dev);
struct net_device *dev = skb->dev;
+ int skb_iif = skb->skb_iif;
struct iphdr *iph;
int err;
@@ -72,6 +73,14 @@ void iptunnel_xmit(struct sock *sk, struct rtable *rt,
struct sk_buff *skb,
skb_dst_set(skb, &rt->dst);
memset(IPCB(skb), 0, sizeof(*IPCB(skb)));
+ if (skb_iif && proto == IPPROTO_UDP) {
+ /* Arrived from an ingress interface and got udp encapuslated.
+ * The encapsulated network segment length may exceed dst mtu.
+ * Allow IP Fragmentation of segments.
+ */
+ IPCB(skb)->flags |= IPSKB_FRAG_SEGS;
+ }
+
/* Push down and install the IP header. */
skb_push(skb, sizeof(struct iphdr));
skb_reset_network_header(skb);
--
2.7.4
