drop_monitor: Make updating data->skb smp safe

Eric Dumazet pointed out to me that the drop_monitor protocol has some holes in
its smp protections.  Specifically, its possible to replace data->skb while its
being written.  This patch corrects that by making data->skb an rcu protected
variable.  That will prevent it from being overwritten while a tracepoint is
modifying it.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: David Miller <davem@davemloft.net>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 54 insertions, 16 deletions

diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c
index a221a5bbecf7..7592943513e3 100644
--- a/net/core/drop_monitor.c
+++ b/net/core/drop_monitor.c
@@ -46,7 +46,7 @@ static DEFINE_MUTEX(trace_state_mutex);
 
 struct per_cpu_dm_data {
         struct work_struct dm_alert_work;
-        struct sk_buff *skb;
+        struct sk_buff __rcu *skb;
         atomic_t dm_hit_count;
         struct timer_list send_timer;
 };
@@ -73,35 +73,58 @@ static int dm_hit_limit = 64;
 static int dm_delay = 1;
 static unsigned long dm_hw_check_delta = 2*HZ;
 static LIST_HEAD(hw_stats_list);
+static int initialized = 0;
 
 static void reset_per_cpu_data(struct per_cpu_dm_data *data)
 {
         size_t al;
         struct net_dm_alert_msg *msg;
         struct nlattr *nla;
+        struct sk_buff *skb;
+        struct sk_buff *oskb = rcu_dereference_protected(data->skb, 1);
 
         al = sizeof(struct net_dm_alert_msg);
         al += dm_hit_limit * sizeof(struct net_dm_drop_point);
         al += sizeof(struct nlattr);
 
-        data->skb = genlmsg_new(al, GFP_KERNEL);
-        genlmsg_put(data->skb, 0, 0, &net_drop_monitor_family,
-                        0, NET_DM_CMD_ALERT);
-        nla = nla_reserve(data->skb, NLA_UNSPEC, sizeof(struct net_dm_alert_msg));
-        msg = nla_data(nla);
-        memset(msg, 0, al);
-        atomic_set(&data->dm_hit_count, dm_hit_limit);
+        skb = genlmsg_new(al, GFP_KERNEL);
+
+        if (skb) {
+                genlmsg_put(skb, 0, 0, &net_drop_monitor_family,
+                                0, NET_DM_CMD_ALERT);
+                nla = nla_reserve(skb, NLA_UNSPEC,
+                                  sizeof(struct net_dm_alert_msg));
+                msg = nla_data(nla);
+                memset(msg, 0, al);
+        } else if (initialized)
+                schedule_work_on(smp_processor_id(), &data->dm_alert_work);
+
+        /*
+         * Don't need to lock this, since we are guaranteed to only
+         * run this on a single cpu at a time.
+         * Note also that we only update data->skb if the old and new skb
+         * pointers don't match.  This ensures that we don't continually call
+         * synchornize_rcu if we repeatedly fail to alloc a new netlink message.
+         */
+        if (skb != oskb) {
+                rcu_assign_pointer(data->skb, skb);
+
+                synchronize_rcu();
+
+                atomic_set(&data->dm_hit_count, dm_hit_limit);
+        }
+
 }
 
 static void send_dm_alert(struct work_struct *unused)
 {
         struct sk_buff *skb;
-        struct per_cpu_dm_data *data = &__get_cpu_var(dm_cpu_data);
+        struct per_cpu_dm_data *data = &get_cpu_var(dm_cpu_data);
 
         /*
          * Grab the skb we're about to send
          */
-        skb = data->skb;
+        skb = rcu_dereference_protected(data->skb, 1);
 
         /*
          * Replace it with a new one
@@ -111,8 +134,10 @@ static void send_dm_alert(struct work_struct *unused)
         /*
          * Ship it!
          */
-        genlmsg_multicast(skb, 0, NET_DM_GRP_ALERT, GFP_KERNEL);
+        if (skb)
+                genlmsg_multicast(skb, 0, NET_DM_GRP_ALERT, GFP_KERNEL);
 
+        put_cpu_var(dm_cpu_data);
 }
 
 /*
@@ -123,9 +148,11 @@ static void send_dm_alert(struct work_struct *unused)
  */
 static void sched_send_work(unsigned long unused)
 {
-        struct per_cpu_dm_data *data =  &__get_cpu_var(dm_cpu_data);
+        struct per_cpu_dm_data *data =  &get_cpu_var(dm_cpu_data);
+
+        schedule_work_on(smp_processor_id(), &data->dm_alert_work);
 
-        schedule_work(&data->dm_alert_work);
+        put_cpu_var(dm_cpu_data);
 }
 
 static void trace_drop_common(struct sk_buff *skb, void *location)
@@ -134,9 +161,16 @@ static void trace_drop_common(struct sk_buff *skb, void *location)
         struct nlmsghdr *nlh;
         struct nlattr *nla;
         int i;
-        struct per_cpu_dm_data *data = &__get_cpu_var(dm_cpu_data);
+        struct sk_buff *dskb;
+        struct per_cpu_dm_data *data = &get_cpu_var(dm_cpu_data);
 
 
+        rcu_read_lock();
+        dskb = rcu_dereference(data->skb);
+
+        if (!dskb)
+                goto out;
+
         if (!atomic_add_unless(&data->dm_hit_count, -1, 0)) {
                 /*
                  * we're already at zero, discard this hit
@@ -144,7 +178,7 @@ static void trace_drop_common(struct sk_buff *skb, void *location)
                 goto out;
         }
 
-        nlh = (struct nlmsghdr *)data->skb->data;
+        nlh = (struct nlmsghdr *)dskb->data;
         nla = genlmsg_data(nlmsg_data(nlh));
         msg = nla_data(nla);
         for (i = 0; i < msg->entries; i++) {
@@ -158,7 +192,7 @@ static void trace_drop_common(struct sk_buff *skb, void *location)
         /*
          * We need to create a new entry
          */
-        __nla_reserve_nohdr(data->skb, sizeof(struct net_dm_drop_point));
+        __nla_reserve_nohdr(dskb, sizeof(struct net_dm_drop_point));
         nla->nla_len += NLA_ALIGN(sizeof(struct net_dm_drop_point));
         memcpy(msg->points[msg->entries].pc, &location, sizeof(void *));
         msg->points[msg->entries].count = 1;
@@ -170,6 +204,8 @@ static void trace_drop_common(struct sk_buff *skb, void *location)
         }
 
 out:
+        rcu_read_unlock();
+        put_cpu_var(dm_cpu_data);
         return;
 }
 
@@ -375,6 +411,8 @@ static int __init init_net_drop_monitor(void)
                 data->send_timer.function = sched_send_work;
         }
 
+        initialized = 1;
+
         goto out;
 
 out_unreg: