[PKT_SCHED] netem: Fix slab corruption with netem (2nd try)

CONFIG_DEBUG_SLAB found the following bug:
netem_enqueue() in sch_netem.c gets a pointer inside a slab object:
struct netem_skb_cb *cb = (struct netem_skb_cb *)skb->cb;
But then, the slab object may be freed:
skb = skb_unshare(skb, GFP_ATOMIC)
cb is still pointing inside the freed skb, so here is a patch to
initialize cb later, and make it clear that initializing it sooner
is a bad idea.

[From Stephen Hemminger: leave cb unitialized in order to let gcc
complain in case of use before initialization]

Signed-off-by: Guillaume Chazarain <guichaz@yahoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 1 deletions

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index c5bd8064e6d..a08ec4c7c55 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -148,7 +148,8 @@ static long tabledist(unsigned long mu, long sigma,
 static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 {
         struct netem_sched_data *q = qdisc_priv(sch);
-        struct netem_skb_cb *cb = (struct netem_skb_cb *)skb->cb;
+        /* We don't fill cb now as skb_unshare() may invalidate it */
+        struct netem_skb_cb *cb;
         struct sk_buff *skb2;
         int ret;
         int count = 1;
@@ -200,6 +201,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch)
                 skb->data[net_random() % skb_headlen(skb)] ^= 1<<(net_random() % 8);
         }
 
+        cb = (struct netem_skb_cb *)skb->cb;
         if (q->gap == 0                 /* not doing reordering */
             || q->counter < q->gap      /* inside last reordering gap */
             || q->reorder < get_crandom(&q->reorder_cor)) {