Subject: Re: [net PATCH] flow_dissector: Fix unaligned access in __skb_flow_dissector when used by eth_get_headlen

From: Sowmini Varadhan <[email-masked]>
Date: Sat, 30 Jan 2016 14:12:20 -0500

On (01/30/16 09:43), Tom Herbert wrote:
> That is not the only case, If a GRE TEB packet is ever received and
> flow dissector is called for any reason (like skb_get_hash) there's
> going to be problems-- and this doesn't require GRE to even be
> configured on the host.
> 
> I have a patch that changes the 32-bit accesses in flow_dissector to
> use get_unaligned_be32. I don't have access to any machines that care
> about alignment (only x86). Do you know if there is an alternate way
> to test this other than running on architecture like Sparc?

I can help test this on sparc (would help if you send me any special
config instructions for the GRE TEB case) , but the other way to test
it would be something similar to this:

--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -178,6 +178,8 @@ ip:
 
                ip_proto = iph->protocol;
 
+               WARN_ON_ONCE(!IS_ALIGNED(iph->saddr, 4));
+
                if (!dissector_uses_key(flow_dissector,
                                        FLOW_DISSECTOR_KEY_IPV4_ADDRS))