[PATCH net] ipv6: make ip6_create_rt_rcu return ip6_null_entry instead of NULL

[Xin Long]

Jianlin reported a crash:

  [  381.484332] BUG: unable to handle kernel NULL pointer dereference at 
0000000000000068
  [  381.619802] RIP: 0010:fib6_rule_lookup+0xa3/0x160
  [  382.009615] Call Trace:
  [  382.020762]  <IRQ>
  [  382.030174]  ip6_route_redirect.isra.52+0xc9/0xf0
  [  382.050984]  ip6_redirect+0xb6/0xf0
  [  382.066731]  icmpv6_notify+0xca/0x190
  [  382.083185]  ndisc_redirect_rcv+0x10f/0x160
  [  382.102569]  ndisc_rcv+0xfb/0x100
  [  382.117725]  icmpv6_rcv+0x3f2/0x520
  [  382.133637]  ip6_input_finish+0xbf/0x460
  [  382.151634]  ip6_input+0x3b/0xb0
  [  382.166097]  ipv6_rcv+0x378/0x4e0

It was caused by the lookup function __ip6_route_redirect() returns NULL in
fib6_rule_lookup() when ip6_create_rt_rcu() returns NULL.

So we fix it by simply making ip6_create_rt_rcu() return ip6_null_entry
instead of NULL.

Fixes: e873e4b9cc7e ("ipv6: use fib6_info_hold_safe() when necessary")
Reported-by: Jianlin Shi <ji...@redhat.com>
Suggested-by: Paolo Abeni <pab...@redhat.com>
Signed-off-by: Xin Long <lucien....@gmail.com>
---
 net/ipv6/route.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 4ef4bbd..754777d 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1040,13 +1040,17 @@ static struct rt6_info *ip6_create_rt_rcu(struct 
fib6_info *rt)
        struct rt6_info *nrt;
 
        if (!fib6_info_hold_safe(rt))
-               return NULL;
+               goto fallback;
 
        nrt = ip6_dst_alloc(dev_net(dev), dev, flags);
-       if (nrt)
+       if (nrt) {
                ip6_rt_copy_init(nrt, rt);
-       else
+       } else {
                fib6_info_release(rt);
+fallback:
+               nrt = dev_net(dev)->ipv6.ip6_null_entry;
+               dst_hold(&nrt->dst);
+       }
 
        return nrt;
 }
@@ -1096,10 +1100,6 @@ static struct rt6_info *ip6_pol_route_lookup(struct net 
*net,
                dst_hold(&rt->dst);
        } else {
                rt = ip6_create_rt_rcu(f6i);
-               if (!rt) {
-                       rt = net->ipv6.ip6_null_entry;
-                       dst_hold(&rt->dst);
-               }
        }
 
        rcu_read_unlock();
-- 
2.1.0