Re: [Xen-devel] [PATCH net-next v2] xen-netfront: clean up code in xennet_release_rx_bufs

[David Vrabel <david.vrabel@citrix.com>]

On 17/01/14 12:08, Wei Liu wrote:
> On Fri, Jan 17, 2014 at 02:25:40PM +0800, annie li wrote:
>>
>> On 2014/1/16 19:10, David Vrabel wrote:
>>> On 15/01/14 23:57, Annie Li wrote:
>>>> This patch implements two things:
>>>>
>>>> * release grant reference and skb for rx path, this fixex resource leaking.
>>>> * clean up grant transfer code kept from old netfront(2.6.18) which grants
>>>> pages for access/map and transfer. But grant transfer is deprecated in current
>>>> netfront, so remove corresponding release code for transfer.
>>>>
>>>> gnttab_end_foreign_access_ref may fail when the grant entry is currently used
>>>> for reading or writing. But this patch does not cover this and improvement for
>>>> this failure may be implemented in a separate patch.
>>> I don't think replacing a resource leak with a security bug is a good idea.
>>>
>>> If you would prefer not to fix the gnttab_end_foreign_access() call, I
>>> think you can fix this in netfront by taking a reference to the page
>>> before calling gnttab_end_foreign_access().  This will ensure the page
>>> isn't freed until the subsequent kfree_skb(), or the gref is released by
>>> the foreign domain (whichever is later).
>>
>> Taking a reference to the page before calling
>> gnttab_end_foreign_access() delays the free work until kfree_skb().
>> Simply adding put_page before kfree_skb() does not make things
>> different from gnttab_end_foreign_access_ref(), and the pages will
>> be freed by kfree_skb(), problem will be hit in
>> gnttab_handle_deferred() when freeing pages which already be freed.
>>
> 
> I think David's idea is:
> 
> 	get_page
> 	gnttab_end_foreign_access
> 	kfree_skb
> 
> The get_page is to offset put_page in gnttab_end_foreign_access. You
> don't need to put page before kfree_skb.

Yes.

David