ipv6: route: purge exception on removal [Linux 5.0]

This Linux kernel change "ipv6: route: purge exception on removal" is included in the Linux 5.0 release. This change is authored by Paolo Abeni <pabeni [at] redhat.com> on Wed Feb 20 18:18:12 2019 +0100. The commit for this change in Linux stable tree is f5b51fe (patch).

ipv6: route: purge exception on removal

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.

v1 -> v2:
 - re-order the code to avoid accessing dst and net after dst_dev_put()

Fixes: 93531c674315 ("net/ipv6: separate handling of FIB entries from dst based routes")
Signed-off-by: Paolo Abeni <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

There are 13 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 net/ipv6/route.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 74b9b6f..047c472 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1274,18 +1274,29 @@ static struct rt6_info *rt6_make_pcpu_route(struct net *net,
 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;

    net = dev_net(rt6_ex->rt6i->dst.dev);
+   net->ipv6.rt6_stats->fib_rt_cache--;
+
+   /* 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);
+
    hlist_del_rcu(&rt6_ex->hlist);
    dst_release(&rt6_ex->rt6i->dst);
    kfree_rcu(rt6_ex, rcu);
    WARN_ON_ONCE(!bucket->depth);
    bucket->depth--;
-   net->ipv6.rt6_stats->fib_rt_cache--;
 }

 /* Remove oldest rt6_ex in bucket and free the memory

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