Re: [PATCH net] ipv6: route: purge exception on removal

[Paolo Abeni <pabeni@redhat.com>]

On Wed, 2019-02-20 at 15:08 +0100, Paolo Abeni wrote:
> When a netdevice is unregistered, we flush the relevant exception
> via rt6_sync_down_dev() -> fib6_ifdown() -> fib6_del() -> fib6_del_route().
> 
> Finally, we end-up calling rt6_remove_exception(), where we release
> the relevant dst, while we keep the references to the related fib6_info and
> dev. Such references should be released later when the dst will be
> destroyed.
> 
> There are a number of caches that can keep the exception around for an
> unlimited amount of time - namely dst_cache, possibly even socket cache.
> As a result device registration may hang, as demonstrated by this script:
> 
> ip netns add cl
> ip netns add rt
> ip netns add srv
> ip netns exec rt sysctl -w net.ipv6.conf.all.forwarding=1
> 
> ip link add name cl_veth type veth peer name cl_rt_veth
> ip link set dev cl_veth netns cl
> ip -n cl link set dev cl_veth up
> ip -n cl addr add dev cl_veth 2001::2/64
> ip -n cl route add default via 2001::1
> 
> ip -n cl link add tunv6 type ip6tnl mode ip6ip6 local 2001::2 remote 2002::1 hoplimit 64 dev cl_veth
> ip -n cl link set tunv6 up
> ip -n cl addr add 2013::2/64 dev tunv6
> 
> ip link set dev cl_rt_veth netns rt
> ip -n rt link set dev cl_rt_veth up
> ip -n rt addr add dev cl_rt_veth 2001::1/64
> 
> ip link add name rt_srv_veth type veth peer name srv_veth
> ip link set dev srv_veth netns srv
> ip -n srv link set dev srv_veth up
> ip -n srv addr add dev srv_veth 2002::1/64
> ip -n srv route add default via 2002::2
> 
> ip -n srv link add tunv6 type ip6tnl mode ip6ip6 local 2002::1 remote 2001::2 hoplimit 64 dev srv_veth
> ip -n srv link set tunv6 up
> ip -n srv addr add 2013::1/64 dev tunv6
> 
> ip link set dev rt_srv_veth netns rt
> ip -n rt link set dev rt_srv_veth up
> ip -n rt addr add dev rt_srv_veth 2002::2/64
> 
> ip netns exec srv netserver & sleep 0.1
> ip netns exec cl ping6 -c 4 2013::1
> ip netns exec cl netperf -H 2013::1 -t TCP_STREAM -l 3 & sleep 1
> ip -n rt link set dev rt_srv_veth mtu 1400
> wait %2
> 
> ip -n cl link del cl_veth
> 
> This commit addresses the issue purging all the references held by the
> exception at time, as we currently do for e.g. ipv6 pcpu dst entries.
> 
> Fixes: 93531c674315 ("net/ipv6: separate handling of FIB entries from dst based routes")
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
>  net/ipv6/route.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index 964491cf3672..43a819ae46c2 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -1274,11 +1274,21 @@ static DEFINE_SPINLOCK(rt6_exception_lock);
>  static void rt6_remove_exception(struct rt6_exception_bucket *bucket,
>  				 struct rt6_exception *rt6_ex)
>  {
> +	struct fib6_info *from;
>  	struct net *net;
>  
>  	if (!bucket || !rt6_ex)
>  		return;
>  
> +	/* purge completely the exception to allow releasing the held resources:
> +	 * some [sk] cache may keep the dst around for unlimited time
> +	 */
> +	from = rcu_dereference_protected(rt6_ex->rt6i->from,
> +					 lockdep_is_held(&rt6_exception_lock));
> +	rcu_assign_pointer(rt6_ex->rt6i->from, NULL);
> +	fib6_info_release(from);
> +	dst_dev_put(&rt6_ex->rt6i->dst);
> +
>  	net = dev_net(rt6_ex->rt6i->dst.dev);

uhm... This is possibly racy, if dev goes away on dst_dev_put(). I'll
send a v2 reordering the operation so that we don't access dev (and net
) after dst_dev_put().

Sorry for the noise,

Paolo