Re: [PATCH net] udp6: fix socket leak on early demux

[Paolo Abeni <pabeni@redhat.com>]

On Thu, 2017-07-27 at 00:06 -0700, Eric Dumazet wrote:
> On Wed, 2017-07-26 at 17:29 +0200, Paolo Abeni wrote:
> > When an early demuxed packet reaches __udp6_lib_lookup_skb(), the
> > sk reference is retrieved and used, but the relevant reference
> > count is leaked and the socket destructor is never called.
> > Beyond leaking the sk memory, if there are pending UDP packets
> > in the receive queue, even the related accounted memory is leaked.
> > 
> > In the long run, this will cause persistent forward allocation errors
> > and no UDP skbs (both ipv4 and ipv6) will be able to reach the
> > user-space.
> > 
> > Fix this by explicitly accessing the early demux reference before
> > the lookup, and properly decreasing the socket reference count
> > after usage.
> > 
> > Also drop the skb_steal_sock() in __udp6_lib_lookup_skb(), and
> > the now obsoleted comment about "socket cache".
> > 
> > The newly added code is derived from the current ipv4 code for the
> > similar path.
> 
> 
> Nice catch Paolo.
> 
> I believe there is one point to discuss, see below.
> 
> > 
> > Reported-by: Sam Edwards <CFSworks@gmail.com>
> > Reported-by: Marc Haber <mh+netdev@zugschlus.de>
> > Fixes: 5425077d73e0 ("net: ipv6: Add early demux handler for UDP unicast")
> > Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> > ---
> >  include/net/udp.h |  1 +
> >  net/ipv4/udp.c    |  3 ++-
> >  net/ipv6/udp.c    | 28 +++++++++++++++++++---------
> >  3 files changed, 22 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/net/udp.h b/include/net/udp.h
> > index 56ce2d2a612d..cc8036987dcb 100644
> > --- a/include/net/udp.h
> > +++ b/include/net/udp.h
> > @@ -260,6 +260,7 @@ static inline struct sk_buff *skb_recv_udp(struct sock *sk, unsigned int flags,
> >  }
> >  
> >  void udp_v4_early_demux(struct sk_buff *skb);
> > +void udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst);
> >  int udp_get_port(struct sock *sk, unsigned short snum,
> >  		 int (*saddr_cmp)(const struct sock *,
> >  				  const struct sock *));
> > diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> > index fac7cb9e3b0f..e6276fa3750b 100644
> > --- a/net/ipv4/udp.c
> > +++ b/net/ipv4/udp.c
> > @@ -1928,7 +1928,7 @@ static int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
> >  /* For TCP sockets, sk_rx_dst is protected by socket lock
> >   * For UDP, we use xchg() to guard against concurrent changes.
> >   */
> > -static void udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
> > +void udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
> >  {
> >  	struct dst_entry *old;
> >  
> > @@ -1937,6 +1937,7 @@ static void udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
> >  		dst_release(old);
> >  	}
> >  }
> > +EXPORT_SYMBOL(udp_sk_rx_dst_set);
> >  
> >  /*
> >   *	Multicasts and broadcasts go to each listener.
> > diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> > index 4a3e65626e8b..e74fe497d823 100644
> > --- a/net/ipv6/udp.c
> > +++ b/net/ipv6/udp.c
> > @@ -291,11 +291,7 @@ static struct sock *__udp6_lib_lookup_skb(struct sk_buff *skb,
> >  					  struct udp_table *udptable)
> >  {
> >  	const struct ipv6hdr *iph = ipv6_hdr(skb);
> > -	struct sock *sk;
> >  
> > -	sk = skb_steal_sock(skb);
> > -	if (unlikely(sk))
> > -		return sk;
> >  	return __udp6_lib_lookup(dev_net(skb->dev), &iph->saddr, sport,
> >  				 &iph->daddr, dport, inet6_iif(skb),
> >  				 udptable, skb);
> > @@ -804,6 +800,25 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
> >  	if (udp6_csum_init(skb, uh, proto))
> >  		goto csum_error;
> >  
> > +	/* Check if the socket is already available, e.g. due to early demux */
> > +	sk = skb_steal_sock(skb);
> > +	if (sk) {
> > +		struct dst_entry *dst = skb_dst(skb);
> > +		int ret;
> > +
> > +		if (unlikely(sk->sk_rx_dst != dst))
> > +			udp_sk_rx_dst_set(sk, dst);
> > +
> > +		ret = udpv6_queue_rcv_skb(sk, skb);
> > +		sock_put(sk);
> > +		/* a return value > 0 means to resubmit the input, but
> > +		 * it wants the return to be -protocol, or 0
> > +		 */
> > +		if (ret > 0)
> > +			return -ret;
> 
> IPv6 and IPv4 have different behavior for resubmit
> 
> I believe "return ret;"  would be more appropriate here.

Thank you for the feedback!

You are very right. I stared at the correct UDPv6 code for unicast
packets, a few lines below, for a lot of time before noticing the
difference :-(

I'll resubmit a v2 soon, thanks!

Paolo