[net] net: fix double free issue of skbuff

If skb_reorder_vlan_header() failed, skb is freed and NULL is returned.
Then at skb_vlan_untag(), it will free skbuff again which cause double
free.

This patch removes kfree_skb() call in function skb_reorder_vlan_header().

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
---
 net/core/skbuff.c | 1 -
 1 file changed, 1 deletion(-)

Hello.

On 2/29/2016 3:22 PM, Zhang Shengju wrote:

> If skb_reorder_vlan_header() failed, skb is freed and NULL is returned.
> Then at skb_vlan_untag(), it will free skbuff again which cause double
> free.
>
> This patch removes kfree_skb() call in function skb_reorder_vlan_header().
>
> Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
> ---
>   net/core/skbuff.c | 1 -
>   1 file changed, 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 488566b..1312d4b 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4350,7 +4350,6 @@ EXPORT_SYMBOL_GPL(skb_gso_transport_seglen);
>   static struct sk_buff *skb_reorder_vlan_header(struct sk_buff *skb)
>   {
>   	if (skb_cow(skb, skb_headroom(skb)) < 0) {
> -		kfree_skb(skb);
>   		return NULL;
>   	}

    You now need to remove {}.

MBR, Sergei

On Mon, 2016-02-29 at 12:22 +0000, Zhang Shengju wrote:
> If skb_reorder_vlan_header() failed, skb is freed and NULL is returned.
> Then at skb_vlan_untag(), it will free skbuff again which cause double
> free.

On skb_reorder_vlan_header() failure, skb_vlan_untag() will call
kfree_skb() using the return value of skb_reorder_vlan_header(), that is
NULL. kfree_skb() is a noop when the argument is NULL.

The current code seams safe.

Paolo

From: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Date: Mon, 29 Feb 2016 12:22:53 +0000

> If skb_reorder_vlan_header() failed, skb is freed and NULL is returned.
> Then at skb_vlan_untag(), it will free skbuff again which cause double
> free.

The 'skb' local variable in this case will be set to "NULL", calling
kfree_skb() on NULL doesn't do anything.

> This patch removes kfree_skb() call in function skb_reorder_vlan_header().
> 
> Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>

Please analyze the complete control path of the caller of this
function, and you'll find that everything is fine.