[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, 18 insertions, 4 deletions

diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 51557c27a0cd..452a2ac4eec8 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -676,6 +676,20 @@ static void ndisc_solicit(struct neighbour *neigh, struct sk_buff *skb)
         }
 }
 
+static struct pneigh_entry *pndisc_check_router(struct net_device *dev,
+                struct in6_addr *addr, int *is_router)
+{
+        struct pneigh_entry *n;
+
+        read_lock_bh(&nd_tbl.lock);
+        n = __pneigh_lookup(&nd_tbl, &init_net, addr, dev);
+        if (n != NULL)
+                *is_router = (n->flags & NTF_ROUTER);
+        read_unlock_bh(&nd_tbl.lock);
+
+        return n;
+}
+
 static void ndisc_recv_ns(struct sk_buff *skb)
 {
         struct nd_msg *msg = (struct nd_msg *)skb_transport_header(skb);
@@ -692,7 +706,7 @@ static void ndisc_recv_ns(struct sk_buff *skb)
         struct pneigh_entry *pneigh = NULL;
         int dad = ipv6_addr_any(saddr);
         int inc;
-        int is_router;
+        int is_router = 0;
 
         if (ipv6_addr_is_multicast(&msg->target)) {
                 ND_PRINTK2(KERN_WARNING
@@ -790,8 +804,8 @@ static void ndisc_recv_ns(struct sk_buff *skb)
                 if (ipv6_chk_acast_addr(dev, &msg->target) ||
                     (idev->cnf.forwarding &&
                      (ipv6_devconf.proxy_ndp || idev->cnf.proxy_ndp) &&
-                     (pneigh = pneigh_lookup(&nd_tbl, &init_net,
-                                             &msg->target, dev, 0)) != NULL)) {
+                     (pneigh = pndisc_check_router(dev, &msg->target,
+                                                  &is_router)) != NULL)) {
                         if (!(NEIGH_CB(skb)->flags & LOCALLY_ENQUEUED) &&
                             skb->pkt_type != PACKET_HOST &&
                             inc != 0 &&
@@ -812,7 +826,7 @@ static void ndisc_recv_ns(struct sk_buff *skb)
                         goto out;
         }
 
-        is_router = !!(pneigh ? pneigh->flags & NTF_ROUTER : idev->cnf.forwarding);
+        is_router = !!(pneigh ? is_router : idev->cnf.forwarding);
 
         if (dad) {
                 struct in6_addr maddr;