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

[Guillaume Chazarain]

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]

Thanks.

--
Guillaume



Signed-off-by: Guillaume Chazarain [EMAIL PROTECTED]
---
 net/sched/sch_netem.c |5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff -r 1b8d63e34819 net/sched/sch_netem.c
--- 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;
 	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)) {

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

[David Miller]

From: Guillaume Chazarain [EMAIL PROTECTED]
Date: Sun, 16 Jul 2006 23:56:31 +0200

 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]

Looks good to me, applied, thanks a lot.
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