nl_fib_input re-reuses incoming skb to send the reply. This means that this
packet will be freed twice, namely in:
- netlink_unicast_kernel
- on receive path
Use clone to send as a cure, the caller is responsible for kfree_skb on error.

Thanks to Alexey Dobryan, who originally found the problem.

Signed-off-by: Denis V. Lunev <den@openvz.org>
---
diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index b03c4c6..ac6238a 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -832,10 +832,13 @@ static void nl_fib_input(struct sk_buff *skb)
 
 	nlh = nlmsg_hdr(skb);
 	if (skb->len < NLMSG_SPACE(0) || skb->len < nlh->nlmsg_len ||
-	    nlh->nlmsg_len < NLMSG_LENGTH(sizeof(*frn))) {
-		kfree_skb(skb);
+	    nlh->nlmsg_len < NLMSG_LENGTH(sizeof(*frn)))
 		return;
-	}
+
+	skb = skb_clone(skb, GFP_KERNEL);
+	if (skb == NULL)
+		return;
+	nlh = nlmsg_hdr(skb);
 
 	frn = (struct fib_result_nl *) NLMSG_DATA(nlh);
 	tb = fib_get_table(frn->tb_id_in);

From: "Denis V. Lunev" <den@openvz.org>
Date: Fri, 21 Dec 2007 12:00:43 +0300

> nl_fib_input re-reuses incoming skb to send the reply. This means that this
> packet will be freed twice, namely in:
> - netlink_unicast_kernel
> - on receive path
> Use clone to send as a cure, the caller is responsible for kfree_skb on error.
> 
> Thanks to Alexey Dobryan, who originally found the problem.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>

What introduced this bug?  This code didn't have this
problem previously.

David Miller wrote:
> From: "Denis V. Lunev" <den@openvz.org>
> Date: Fri, 21 Dec 2007 12:00:43 +0300
> 
>> nl_fib_input re-reuses incoming skb to send the reply. This means that this
>> packet will be freed twice, namely in:
>> - netlink_unicast_kernel
>> - on receive path
>> Use clone to send as a cure, the caller is responsible for kfree_skb on error.
>>
>> Thanks to Alexey Dobryan, who originally found the problem.
>>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
> 
> What introduced this bug?  This code didn't have this
> problem previously.
> 

commit cd40b7d3983c708aabe3d3008ec64ffce56d33b0
Author: Denis V. Lunev <den@openvz.org>
Date:   Wed Oct 10 21:15:29 2007 -0700

From: "Denis V. Lunev" <den@sw.ru>
Date: Fri, 21 Dec 2007 12:39:36 +0300

> David Miller wrote:
> > What introduced this bug?  This code didn't have this
> > problem previously.
> 
> commit cd40b7d3983c708aabe3d3008ec64ffce56d33b0
> Author: Denis V. Lunev <den@openvz.org>
> Date:   Wed Oct 10 21:15:29 2007 -0700

Thank you, I'm going to add this reference to the changeset
comments so that it will help others who look at this fix.

Consider your patch applied, thanks! :-)