[net] net: fix double free issue of skbuff

[net] net: fix double free issue of skbuff

[Zhang Shengju]

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;
 	}
 
-- 
1.8.3.1

Re: [net] net: fix double free issue of skbuff

[Sergei Shtylyov]

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

Re: [net] net: fix double free issue of skbuff

[Paolo Abeni]

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

Re: [net] net: fix double free issue of skbuff

[David Miller]

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.

Re: [net] net: fix double free issue of skbuff

[张胜举]

> 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

Thanks Sergei, I will add this in v2. 

BRs,
Shengju

Re: [net] net: fix double free issue of skbuff

[张胜举]

> 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
Hi Paolo, even current code is safe, this's still a potential problem. We should make an
assumption that inner function doesn't free skb, and let outside function take care of this.

BRs, 
Shengju

Re: [net] net: fix double free issue of skbuff

[David Miller]

From: 张胜举 <zhangshengju@cmss.chinamobile.com>
Date: Mon, 29 Feb 2016 22:16:37 +0800

>> 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
> Hi Paolo, even current code is safe, this's still a potential problem. We should make an
> assumption that inner function doesn't free skb, and let outside function take care of this.

No, the current code is intentional and perfectly fine.

Fix real bugs, not imaginary ones.

Thanks.