From: Florian Westphal fw@strlen.de
[ Upstream commit bb4cc1a18856a73f0ff5137df0c2a31f4c50f6cf ]
Conntrack reassembly records the largest fragment size seen in IPCB. However, when this gets forwarded/transmitted, fragmentation will only be forced if one of the fragmented packets had the DF bit set.
In that case, a flag in IPCB will force fragmentation even if the MTU is large enough.
This should work fine, but this breaks with ip tunnels. Consider client that sends a UDP datagram of size X to another host.
The client fragments the datagram, so two packets, of size y and z, are sent. DF bit is not set on any of these packets.
Middlebox netfilter reassembles those packets back to single size-X packet, before routing decision.
packet-size-vs-mtu checks in ip_forward are irrelevant, because DF bit isn't set. At output time, ip refragmentation is skipped as well because x is still smaller than the mtu of the output device.
If ttransmit device is an ip tunnel, the packet size increases to x+overhead.
Also, tunnel might be configured to force DF bit on outer header.
In this case, packet will be dropped (exceeds MTU) and an ICMP error is generated back to sender.
But sender already respects the announced MTU, all the packets that it sent did fit the announced mtu.
Force refragmentation as per original sizes unconditionally so ip tunnel will encapsulate the fragments instead.
The only other solution I see is to place ip refragmentation in the ip_tunnel code to handle this case.
Fixes: d6b915e29f4ad ("ip_fragment: don't forward defragmented DF packet") Reported-by: Christian Perle christian.perle@secunet.com Signed-off-by: Florian Westphal fw@strlen.de Acked-by: Pablo Neira Ayuso pablo@netfilter.org Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv4/ip_output.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/net/ipv4/ip_output.c +++ b/net/ipv4/ip_output.c @@ -303,7 +303,7 @@ static int __ip_finish_output(struct net if (skb_is_gso(skb)) return ip_finish_output_gso(net, sk, skb, mtu);
- if (skb->len > mtu || (IPCB(skb)->flags & IPSKB_FRAG_PMTU)) + if (skb->len > mtu || IPCB(skb)->frag_max_size) return ip_fragment(net, sk, skb, mtu, ip_finish_output2);
return ip_finish_output2(net, sk, skb);