net: fix NULL dereferences in check_peer_redir()

Gergely Kalman reported crashes in check_peer_redir().

It appears commit f39925dbde778 (ipv4: Cache learned redirect
information in inetpeer.) added a race, leading to possible NULL ptr
dereference.

Since we can now change dst neighbour, we should make sure a reader can
safely use a neighbour.

Add RCU protection to dst neighbour, and make sure check_peer_redir()
can be called safely by different cpus in parallel.

As neighbours are already freed after one RCU grace period, this patch
should not add typical RCU penalty (cache cold effects)

Many thanks to Gergely for providing a pretty report pointing to the
bug.

Reported-by: Gergely Kalman <synapse@hippy.csoma.elte.hu>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 16 insertions, 8 deletions

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index ccaaa851ab42..77d3eded665a 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -204,9 +204,15 @@ static inline int ip_finish_output2(struct sk_buff *skb)
                 skb = skb2;
         }
 
+        rcu_read_lock();
         neigh = dst_get_neighbour(dst);
-        if (neigh)
-                return neigh_output(neigh, skb);
+        if (neigh) {
+                int res = neigh_output(neigh, skb);
+
+                rcu_read_unlock();
+                return res;
+        }
+        rcu_read_unlock();
 
         if (net_ratelimit())
                 printk(KERN_DEBUG "ip_finish_output2: No header cache and no neighbour!\n");
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 1730689f560e..6afc4eb50591 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1628,16 +1628,18 @@ static int check_peer_redir(struct dst_entry *dst, struct inet_peer *peer)
 {
         struct rtable *rt = (struct rtable *) dst;
         __be32 orig_gw = rt->rt_gateway;
-        struct neighbour *n;
+        struct neighbour *n, *old_n;
 
         dst_confirm(&rt->dst);
 
-        neigh_release(dst_get_neighbour(&rt->dst));
-        dst_set_neighbour(&rt->dst, NULL);
-
         rt->rt_gateway = peer->redirect_learned.a4;
-        rt_bind_neighbour(rt);
-        n = dst_get_neighbour(&rt->dst);
+
+        n = ipv4_neigh_lookup(&rt->dst, &rt->rt_gateway);
+        if (IS_ERR(n))
+                return PTR_ERR(n);
+        old_n = xchg(&rt->dst._neighbour, n);
+        if (old_n)
+                neigh_release(old_n);
         if (!n || !(n->nud_state & NUD_VALID)) {
                 if (n)
                         neigh_event_send(n, NULL);