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>

1 files changed, 13 insertions, 4 deletions

diff --git a/include/net/dst.h b/include/net/dst.h
index 29e255796ce1..13d507d69ddb 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -37,7 +37,7 @@ struct dst_entry {
         unsigned long           _metrics;
         unsigned long           expires;
         struct dst_entry        *path;
-        struct neighbour        *_neighbour;
+        struct neighbour __rcu  *_neighbour;
 #ifdef CONFIG_XFRM
         struct xfrm_state       *xfrm;
 #else
@@ -88,12 +88,17 @@ struct dst_entry {
 
 static inline struct neighbour *dst_get_neighbour(struct dst_entry *dst)
 {
-        return dst->_neighbour;
+        return rcu_dereference(dst->_neighbour);
+}
+
+static inline struct neighbour *dst_get_neighbour_raw(struct dst_entry *dst)
+{
+        return rcu_dereference_raw(dst->_neighbour);
 }
 
 static inline void dst_set_neighbour(struct dst_entry *dst, struct neighbour *neigh)
 {
-        dst->_neighbour = neigh;
+        rcu_assign_pointer(dst->_neighbour, neigh);
 }
 
 extern u32 *dst_cow_metrics_generic(struct dst_entry *dst, unsigned long old);
@@ -382,8 +387,12 @@ static inline void dst_rcu_free(struct rcu_head *head)
 static inline void dst_confirm(struct dst_entry *dst)
 {
         if (dst) {
-                struct neighbour *n = dst_get_neighbour(dst);
+                struct neighbour *n;
+
+                rcu_read_lock();
+                n = dst_get_neighbour(dst);
                 neigh_confirm(n);
+                rcu_read_unlock();
         }
 }