Subject: Re: rare bad TCP checksum with 2.6.19?

From: Michael Tokarev <[email-masked]>
Date: Wed, 17 Jan 2007 17:12:47 +0300

Herbert Xu wrote:
> On Tue, Jan 16, 2007 at 11:08:51AM +0300, Michael Tokarev wrote:
>> Ok.  Here's another trace, from that remote network that triggers
>> this thing more-or-less reliable (every 2nd transfer at least) --
>> http://www.corpit.ru/mjt/bh-bad-cksum-dmp.bin . It's a full session
>> between 216.168.29.244 - the requesting/receiving side -- and
>> 81.13.94.6 -- our sending side (the file being transferred is some
>> trojan horse I found on a friend's PC, so be careful ;)
> 
> I'll have a look at this tomorrow.
> 
> Since you're certain that this is being seen on the wire, one
> possibility is that we've got a bug somewhere that's zeroing
> skb->ip_summed on a packet with a partial checksum.

Here's another sample, which may be more useful.  I've seen quite
alot of very similar stuff while running tcpdump.

  http://www.corpit.ru/mjt/bad-cksum-session3-dmp.bin

The scenario looks like this.

A client (82.84.172.37 -- a zombie machine trying to send us spam
in this case) connects to a port 25 here (81.13.94.6:25).  SYN+ACK
sequence completes.  Next, our server send an initial SMTP greething
message, but almost right after that, the client sends a FIN packet,
WITHOUT acknowleging that it received the (first and only) data
packet.  So some time later our machine re-sends the data, AND adds
FIN flag to the packet (also replying to the FIN received from the
client).  And *that* packet - original data packet which is modified
to also include FIN - has incorrect checksum.

So it looks like the checksum isn't being updated WHEN ADDING MORE
FLAGS to the original data packet.

/mjt