Subject: Re: [FYI] xfrm: Don't lookup sk_policy for timewait sockets

From: Sebastian Poehn <[email-masked]>
Date: Thu, 09 Apr 2015 11:24:28 +0200

On Thu, 2015-04-09 at 02:07 -0700, Eric Dumazet wrote:
> On Thu, 2015-04-09 at 10:09 +0200, Sebastian Poehn wrote:
> > We are running a couple of thousand machines with 3.8 and 3.12. On very few systems
> > (something below 10) we encounter panics in xfrm code. The main characteristic seams
> > to be the usage of TPROXY.
> > 
> > Attached patch is only a workaround, as problems may also happen in other code portions
> > (actually on even fewer systems this happens).
> > 
> > For timewait sockets the memory region of sk_policy does not belong
> > to us anymore. So there may be someone else using it and we may panic
> > because of corrupted pointers.
> > 
> > xfrm_sk_policy_lookup+0x38/0x66
> > xfrm_lookup+0x93/0x48f
> > nf_nat_packet+0x92/0xa4 [nf_nat]
> > _decode_session4+0xd9/0x294
> > nf_xfrm_me_harder+0x50/0xc5 [nf_nat]
> > nf_nat_ipv4_out+0xad/0xc4 [iptable_nat]
> > nf_iterate+0x42/0x7d
> > ip_finish_output2+0x2b1/0x2b1
> > nf_hook_slow+0x22f/0x2c9
> > ip_finish_output2+0x2b1/0x2b1
> > ip_finish_output2+0x2b1/0x2b1
> > __xfrm_route_forward+0x7a/0x97
> > ip_finish_output2+0x2b1/0x2b1
> > NF_HOOK_COND+0x3f/0x54
> > ip_output+0x5a/0x5e
> > __netif_receive_skb+0x4b2/0x514
> > process_backlog+0xee/0x1c5
> > net_rx_action+0xa7/0x1fe
> > 
> > Signed-off-by: Sebastian Poehn <[email-masked]>
> > ---
> >  net/xfrm/xfrm_policy.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
> > index 9c6b1ab..e9a74fa 100644
> > --- a/net/xfrm/xfrm_policy.c
> > +++ b/net/xfrm/xfrm_policy.c
> > @@ -2072,7 +2072,7 @@ restart:
> >  	xdst = NULL;
> >  	route = NULL;
> >  
> > -	if (sk && sk->sk_policy[XFRM_POLICY_OUT]) {
> > +	if (sk && sk->sk_state != TCP_TIME_WAIT && sk->sk_policy[XFRM_POLICY_OUT]) {
> >  		num_pols = 1;
> >  		pols[0] = xfrm_sk_policy_lookup(sk, XFRM_POLICY_OUT, fl);
> >  		err = xfrm_expand_policies(fl, family, pols,
> > @@ -2349,7 +2349,7 @@ int __xfrm_policy_check(struct sock *sk, int dir, struct sk_buff *skb,
> >  	}
> >  
> >  	pol = NULL;
> > -	if (sk && sk->sk_policy[dir]) {
> > +	if (sk && sk->sk_state != TCP_TIME_WAIT && sk->sk_policy[dir]) {
> >  		pol = xfrm_sk_policy_lookup(sk, dir, &fl);
> >  		if (IS_ERR(pol)) {
> >  			XFRM_INC_STATS(net, LINUX_MIB_XFRMINPOLERROR);
> 
> Hmm.. interesting.
> 
> TCP stack never sends packets attached to a socket in timewait state.
> 
> On IPv4, the ACK packets sent on behalf of TIME_WAIT follow this path :
> 
> tcp_v4_timewait_ack()
>  tcp_v4_send_ack()
>   ip_send_unicast_reply(*this_cpu_ptr(net->ipv4.tcp_sk))
> 
> The per cpu socket used to attach these skb are not in TCP_TIME_WAIT
> state.
> 
> TPROXY can handle a socket in TCP_TIME_WAIT state only in input path.
> 
> So I am a bit confused. Could you elaborate ?

The setup is a usual transparent proxy one. So TPROXY intercepts the
first connection packet and later on we use socket match to direct to
the local IP_TRANSPARENT socket. So sorry for the confusion - xt_socket
was meant.

The strange thing I noticed is that tw_transparent was 0 (we don't use
xt_socket --transparent). But then I wonder how the tuple can match.

I will collect some more information and post them.
> 
> In any case, you probably should use sk_fullsock() new helper, as I
> presume you'll have a similar issue with TCP_NEW_SYN_RECV pseudo sockets
> when I am done with tcp listener refactoring.

I already saw and like sk_fullsock.
> Thanks.