From: Cyril Strejc cyril.strejc@skoda.cz
[ Upstream commit 9122a70a6333705c0c35614ddc51c274ed1d3637 ]
During a testing of an user-space application which transmits UDP multicast datagrams and utilizes multicast routing to send the UDP datagrams out of defined network interfaces, I've found a multicast router does not fill-in UDP checksum into locally produced, looped-back and forwarded UDP datagrams, if an original output NIC the datagrams are sent to has UDP TX checksum offload enabled.
The datagrams are sent malformed out of the NIC the datagrams have been forwarded to.
It is because:
1. If TX checksum offload is enabled on the output NIC, UDP checksum is not calculated by kernel and is not filled into skb data.
2. dev_loopback_xmit(), which is called solely by ip_mc_finish_output(), sets skb->ip_summed = CHECKSUM_UNNECESSARY unconditionally.
3. Since 35fc92a9 ("[NET]: Allow forwarding of ip_summed except CHECKSUM_COMPLETE"), the ip_summed value is preserved during forwarding.
4. If ip_summed != CHECKSUM_PARTIAL, checksum is not calculated during a packet egress.
The minimum fix in dev_loopback_xmit():
1. Preserves skb->ip_summed CHECKSUM_PARTIAL. This is the case when the original output NIC has TX checksum offload enabled. The effects are:
a) If the forwarding destination interface supports TX checksum offloading, the NIC driver is responsible to fill-in the checksum.
b) If the forwarding destination interface does NOT support TX checksum offloading, checksums are filled-in by kernel before skb is submitted to the NIC driver.
c) For local delivery, checksum validation is skipped as in the case of CHECKSUM_UNNECESSARY, thanks to skb_csum_unnecessary().
2. Translates ip_summed CHECKSUM_NONE to CHECKSUM_UNNECESSARY. It means, for CHECKSUM_NONE, the behavior is unmodified and is there to skip a looped-back packet local delivery checksum validation.
Signed-off-by: Cyril Strejc cyril.strejc@skoda.cz Reviewed-by: Willem de Bruijn willemb@google.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Sasha Levin sashal@kernel.org --- include/net/udp.h | 5 +++-- net/core/dev.c | 3 ++- 2 files changed, 5 insertions(+), 3 deletions(-)
diff --git a/include/net/udp.h b/include/net/udp.h index 360df454356cb..909ecf447e0fb 100644 --- a/include/net/udp.h +++ b/include/net/udp.h @@ -494,8 +494,9 @@ static inline struct sk_buff *udp_rcv_segment(struct sock *sk, * CHECKSUM_NONE in __udp_gso_segment. UDP GRO indeed builds partial * packets in udp_gro_complete_segment. As does UDP GSO, verified by * udp_send_skb. But when those packets are looped in dev_loopback_xmit - * their ip_summed is set to CHECKSUM_UNNECESSARY. Reset in this - * specific case, where PARTIAL is both correct and required. + * their ip_summed CHECKSUM_NONE is changed to CHECKSUM_UNNECESSARY. + * Reset in this specific case, where PARTIAL is both correct and + * required. */ if (skb->pkt_type == PACKET_LOOPBACK) skb->ip_summed = CHECKSUM_PARTIAL; diff --git a/net/core/dev.c b/net/core/dev.c index 9cb47618d4869..2015074d8d9ab 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -3995,7 +3995,8 @@ int dev_loopback_xmit(struct net *net, struct sock *sk, struct sk_buff *skb) skb_reset_mac_header(skb); __skb_pull(skb, skb_network_offset(skb)); skb->pkt_type = PACKET_LOOPBACK; - skb->ip_summed = CHECKSUM_UNNECESSARY; + if (skb->ip_summed == CHECKSUM_NONE) + skb->ip_summed = CHECKSUM_UNNECESSARY; WARN_ON(!skb_dst(skb)); skb_dst_force(skb); netif_rx_ni(skb);