ipv4: fix a race in update_or_create_fnhe()

nh_exceptions is effectively used under rcu, but lacks proper
barriers. Between kzalloc() and setting of nh->nh_exceptions(),
we need a proper memory barrier.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 4895c771c7f00 ("ipv4: Add FIB nexthop exceptions.")
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 8 insertions, 6 deletions

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index b10cd43a4722..5b6efb3d2308 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -157,9 +157,12 @@ static void rt_fibinfo_free(struct rtable __rcu **rtp)
 
 static void free_nh_exceptions(struct fib_nh *nh)
 {
-        struct fnhe_hash_bucket *hash = nh->nh_exceptions;
+        struct fnhe_hash_bucket *hash;
         int i;
 
+        hash = rcu_dereference_protected(nh->nh_exceptions, 1);
+        if (!hash)
+                return;
         for (i = 0; i < FNHE_HASH_SIZE; i++) {
                 struct fib_nh_exception *fnhe;
 
@@ -205,8 +208,7 @@ static void free_fib_info_rcu(struct rcu_head *head)
         change_nexthops(fi) {
                 if (nexthop_nh->nh_dev)
                         dev_put(nexthop_nh->nh_dev);
-                if (nexthop_nh->nh_exceptions)
-                        free_nh_exceptions(nexthop_nh);
+                free_nh_exceptions(nexthop_nh);
                 rt_fibinfo_free_cpus(nexthop_nh->nh_pcpu_rth_output);
                 rt_fibinfo_free(&nexthop_nh->nh_rth_input);
         } endfor_nexthops(fi);
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index eaa4b000c7b4..44b0cbdd76f1 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -628,12 +628,12 @@ static void update_or_create_fnhe(struct fib_nh *nh, __be32 daddr, __be32 gw,
 
         spin_lock_bh(&fnhe_lock);
 
-        hash = nh->nh_exceptions;
+        hash = rcu_dereference(nh->nh_exceptions);
         if (!hash) {
                 hash = kzalloc(FNHE_HASH_SIZE * sizeof(*hash), GFP_ATOMIC);
                 if (!hash)
                         goto out_unlock;
-                nh->nh_exceptions = hash;
+                rcu_assign_pointer(nh->nh_exceptions, hash);
         }
 
         hash += hval;
@@ -1242,7 +1242,7 @@ static unsigned int ipv4_mtu(const struct dst_entry *dst)
 
 static struct fib_nh_exception *find_exception(struct fib_nh *nh, __be32 daddr)
 {
-        struct fnhe_hash_bucket *hash = nh->nh_exceptions;
+        struct fnhe_hash_bucket *hash = rcu_dereference(nh->nh_exceptions);
         struct fib_nh_exception *fnhe;
         u32 hval;