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Subject[ 108/127] drop_monitor: Make updating data->skb smp safe
Date
3.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Neil Horman <nhorman@tuxdriver.com>

commit 3885ca785a3618593226687ced84f3f336dc3860 upstream.

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>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
net/core/drop_monitor.c | 70 +++++++++++++++++++++++++++++++++++++-----------
1 file changed, 54 insertions(+), 16 deletions(-)

--- 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_st
/*
* 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_st
*/
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_
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_
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++) {
@@ -157,7 +191,7 @@ static void trace_drop_common(struct sk_
/*
* 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;
@@ -169,6 +203,8 @@ static void trace_drop_common(struct sk_
}

out:
+ rcu_read_unlock();
+ put_cpu_var(dm_cpu_data);
return;
}

@@ -374,6 +410,8 @@ static int __init init_net_drop_monitor(
data->send_timer.function = sched_send_work;
}

+ initialized = 1;
+
goto out;

out_unreg:



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