Re: [PATCH net] flow_dissector: properly cap thoff field

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 17 Jan 2018 14:21:13 -0800

> From: Eric Dumazet <edumazet@google.com>
> 
> syzbot reported yet another crash [1] that is caused by
> insufficient validation of DODGY packets.
> 
> Two bugs are happening here to trigger the crash.
> 
> 1) Flow dissection leaves with incorrect thoff field.
> 
> 2) skb_probe_transport_header() sets transport header to this invalid
> thoff, even if pointing after skb valid data.
> 
> 3) qdisc_pkt_len_init() reads out-of-bound data because it
> trusts tcp_hdrlen(skb)  
> 
> Possible fixes :
> 
> - Full flow dissector validation before injecting bad DODGY packets in
> the stack.
>  This approach was attempted here : https://patchwork.ozlabs.org/patch/
> 861874/
> 
> - Have more robust functions in the core.
>   This might be needed anyway for stable versions.
> 
> This patch fixes the flow dissection issue.
> 
> [1]
 ...
> Fixes: 34fad54c2537 ("net: __skb_flow_dissect() must cap its return value")
> Fixes: a6e544b0a88b ("flow_dissector: Jump to exit code in __skb_flow_dissect")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Willem de Bruijn <willemb@google.com>
> Reported-by: syzbot <syzkaller@googlegroups.com>

Applied and queued up for -stable, thanks Eric.