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Subject[PATCH 3.2 091/106] ipv6: avoid unregistering inet6_dev for loopback
3.2.93-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: WANG Cong <xiyou.wangcong@gmail.com>

commit 60abc0be96e00ca71bac083215ac91ad2e575096 upstream.

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>
[bwh: Backported to 3.2: the NETDEV_CHANGEMTU case used to fall-through to the
NETDEV_DOWN case here, so replace that with a separate call to addrconf_ifdown()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -2633,6 +2633,7 @@ static int addrconf_notify(struct notifi
{
struct net_device *dev = (struct net_device *) data;
struct inet6_dev *idev = __in6_dev_get(dev);
+ struct net *net = dev_net(dev);
int run_pending = 0;
int err;

@@ -2733,7 +2734,7 @@ static int addrconf_notify(struct notifi
* 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;

@@ -2754,6 +2755,8 @@ static int addrconf_notify(struct notifi
* MTU falled under IPV6_MIN_MTU.
* Stop IPv6 on this interface.
*/
+ addrconf_ifdown(dev, dev != net->loopback_dev);
+ break;

case NETDEV_DOWN:
case NETDEV_UNREGISTER:
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