net: tracepoint of net_dev_xmit sees freed skb and causes panic

Because there is a possibility that skb is kfree_skb()ed and zero cleared
after ndo_start_xmit, we should not see the contents of skb like skb->len and
skb->dev->name after ndo_start_xmit. But trace_net_dev_xmit does that
and causes panic by NULL pointer dereference.
This patch fixes trace_net_dev_xmit not to see the contents of skb directly.

If you want to reproduce this panic,

1. Get tracepoint of net_dev_xmit on
2. Create 2 guests on KVM
2. Make 2 guests use virtio_net
4. Execute netperf from one to another for a long time as a network burden
5. host will panic(It takes about 30 minutes)

Signed-off-by: Koki Sanagi <sanagi.koki@jp.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 12 insertions, 7 deletions

diff --git a/include/trace/events/net.h b/include/trace/events/net.h
index 5f247f5ffc56..f99645d05a8f 100644
--- a/include/trace/events/net.h
+++ b/include/trace/events/net.h
@@ -12,22 +12,24 @@
 TRACE_EVENT(net_dev_xmit,
 
         TP_PROTO(struct sk_buff *skb,
-                 int rc),
+                 int rc,
+                 struct net_device *dev,
+                 unsigned int skb_len),
 
-        TP_ARGS(skb, rc),
+        TP_ARGS(skb, rc, dev, skb_len),
 
         TP_STRUCT__entry(
                 __field(        void *,         skbaddr         )
                 __field(        unsigned int,   len             )
                 __field(        int,            rc              )
-                __string(       name,           skb->dev->name  )
+                __string(       name,           dev->name       )
         ),
 
         TP_fast_assign(
                 __entry->skbaddr = skb;
-                __entry->len = skb->len;
+                __entry->len = skb_len;
                 __entry->rc = rc;
-                __assign_str(name, skb->dev->name);
+                __assign_str(name, dev->name);
         ),
 
         TP_printk("dev=%s skbaddr=%p len=%u rc=%d",
diff --git a/net/core/dev.c b/net/core/dev.c
index c7e305d13b71..939307891e71 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2096,6 +2096,7 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 {
         const struct net_device_ops *ops = dev->netdev_ops;
         int rc = NETDEV_TX_OK;
+        unsigned int skb_len;
 
         if (likely(!skb->next)) {
                 u32 features;
@@ -2146,8 +2147,9 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
                         }
                 }
 
+                skb_len = skb->len;
                 rc = ops->ndo_start_xmit(skb, dev);
-                trace_net_dev_xmit(skb, rc);
+                trace_net_dev_xmit(skb, rc, dev, skb_len);
                 if (rc == NETDEV_TX_OK)
                         txq_trans_update(txq);
                 return rc;
@@ -2167,8 +2169,9 @@ gso:
                 if (dev->priv_flags & IFF_XMIT_DST_RELEASE)
                         skb_dst_drop(nskb);
 
+                skb_len = nskb->len;
                 rc = ops->ndo_start_xmit(nskb, dev);
-                trace_net_dev_xmit(nskb, rc);
+                trace_net_dev_xmit(nskb, rc, dev, skb_len);
                 if (unlikely(rc != NETDEV_TX_OK)) {
                         if (rc & ~NETDEV_TX_MASK)
                                 goto out_kfree_gso_skb;