[NEIGH]: Fix race between pneigh deletion and ipv6's ndisc_recv_ns (v3).

Proxy neighbors do not have any reference counting, so any caller
of pneigh_lookup (unless it's a netlink triggered add/del routine)
should _not_ perform any actions on the found proxy entry. 

There's one exception from this rule - the ipv6's ndisc_recv_ns() 
uses found entry to check the flags for NTF_ROUTER.

This creates a race between the ndisc and pneigh_delete - after 
the pneigh is returned to the caller, the nd_tbl.lock is dropped 
and the deleting procedure may proceed.

One of the fixes would be to add a reference counting, but this
problem exists for ndisc only. Besides such a patch would be too 
big for -rc4.

So I propose to introduce a __pneigh_lookup() which is supposed
to be called with the lock held and use it in ndisc code to check
the flags on alive pneigh entry.


Changes from v2:
As David noticed, Exported the __pneigh_lookup() to ipv6 module. 
The checkpatch generates a warning on it, since the EXPORT_SYMBOL 
does not follow the symbol itself, but in this file all the 
exports come at the end, so I decided no to break this harmony.

Changes from v1:
Fixed comments from YOSHIFUJI - indentation of prototype in header
and the pndisc_check_router() name - and a compilation fix, pointed
by Daniel - the is_routed was (falsely) considered as uninitialized
by gcc.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index ebbfb509822e..64a5f0120b52 100644
--- a/include/net/neighbour.h
+++ b/include/net/neighbour.h
@@ -218,6 +218,10 @@ extern unsigned long		neigh_rand_reach_time(unsigned long base);
 extern void			pneigh_enqueue(struct neigh_table *tbl, struct neigh_parms *p,
 					       struct sk_buff *skb);
 extern struct pneigh_entry	*pneigh_lookup(struct neigh_table *tbl, struct net *net, const void *key, struct net_device *dev, int creat);
+extern struct pneigh_entry	*__pneigh_lookup(struct neigh_table *tbl,
+						 struct net *net,
+						 const void *key,
+						 struct net_device *dev);
 extern int			pneigh_delete(struct neigh_table *tbl, struct net *net, const void *key, struct net_device *dev);
 
 extern void neigh_app_ns(struct neighbour *n);