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

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Date:	Fri, 17 Jan 2014 14:58:30 +0800
From:	annie li <annie.li@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	wei.liu2@...rix.com, ian.campbell@...rix.com,
	netdev@...r.kernel.org, xen-devel@...ts.xen.org,
	andrew.bennieston@...rix.com, davem@...emloft.net
Subject: Re: [Xen-devel] [PATCH net-next v2] xen-netfront: clean up code in
 xennet_release_rx_bufs


On 2014/1/17 14:25, 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.
>
> So put_page is required in gnttab_end_foreign_access(), this will 
> ensure either free is taken by kfree_skb or gnttab_handle_deferred. 
> This involves changes in blkfront/pcifront/tpmfront(just like your 
> patch), this way ensure page is released when ref is end.
>
> Another solution I am thinking is calling gnttab_end_foreign_access() 
> with page parameter as NULL, then gnttab_end_foreign_access will only 
> do ending grant reference work and releasing page work is done by 
> kfree_skb().

Not NULL above, it should be 0UL.

Thanks
Annie

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