ipv6: avoid unregistering inet6_dev for loopback

The per netns loopback_dev->ip6_ptr is unregistered and set to
NULL when its mtu is set to smaller than IPV6_MIN_MTU, this
leads to that we could set rt->rt6i_idev NULL after a
rt6_uncached_list_flush_dev() and then crash after another
call.

In this case we should just bring its inet6_dev down, rather
than unregistering it, at least prior to commit 176c39af29bc
("netns: fix addrconf_ifdown kernel panic") we always
override the case for loopback.

Thanks a lot to Andrey for finding a reliable reproducer.

Fixes: 176c39af29bc ("netns: fix addrconf_ifdown kernel panic")
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Daniel Lezcano <dlezcano@fr.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: David Ahern <dsahern@gmail.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 2 deletions

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 686c92375e81..1d2dbace42ff 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -3369,6 +3369,7 @@ static int addrconf_notify(struct notifier_block *this, unsigned long event,
         struct net_device *dev = netdev_notifier_info_to_dev(ptr);
         struct netdev_notifier_changeupper_info *info;
         struct inet6_dev *idev = __in6_dev_get(dev);
+        struct net *net = dev_net(dev);
         int run_pending = 0;
         int err;
 
@@ -3384,7 +3385,7 @@ static int addrconf_notify(struct notifier_block *this, unsigned long event,
         case NETDEV_CHANGEMTU:
                 /* if MTU under IPV6_MIN_MTU stop IPv6 on this interface. */
                 if (dev->mtu < IPV6_MIN_MTU) {
-                        addrconf_ifdown(dev, 1);
+                        addrconf_ifdown(dev, dev != net->loopback_dev);
                         break;
                 }
 
@@ -3500,7 +3501,7 @@ static int addrconf_notify(struct notifier_block *this, unsigned long event,
                          * IPV6_MIN_MTU stop IPv6 on this interface.
                          */
                         if (dev->mtu < IPV6_MIN_MTU)
-                                addrconf_ifdown(dev, 1);
+                                addrconf_ifdown(dev, dev != net->loopback_dev);
                 }
                 break;