[PATCH] Fix slab corruption with netem

[Guillaume Chazarain <guichaz@yahoo.fr>]

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Hello,

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.

Thanks.

-- 
Guillaume


[-- Attachment #2: netem_cb_initialization.diff --]
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--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -148,7 +148,8 @@ static int netem_enqueue(struct sk_buff 
 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 = NULL;
 	struct sk_buff *skb2;
 	int ret;
 	int count = 1;
@@ -200,6 +201,7 @@ static int netem_enqueue(struct sk_buff 
 		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)) {

-- 
Signed-off-by: Guillaume Chazarain <guichaz@yahoo.fr>