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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Jan 17, 2014 at 08:32:29PM +0800, annie li wrote:
> 
> On 2014-1-17 20:08, 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, this is what I described as following about David's patch.
> 
> >>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.
> 
> But this would has some issue in netfront tx path. Netfront ends all

What issue with tx path? Your patch only touches rx skbs, doesn't it?

> grant reference of one skb first and then release this skb. If the
> gnttab_end_foreign_access_ref fails in gnttab_end_foreign_access(),
> this frag page and corresponding grant reference will be put in
> entry and release work will be done in the timer routine. If some

I understand up to this point.

> frag pages of one skb is free in this timer routine, then
> dev_kfree_skb_irq will free pages which have been freed.

Why is dev_kfree_skb_irq involved? It is used in tx path not rx path.
Even if we look at dev_kfree_skb_irq, it calls __kfree_skb for dropped
packet eventually, which should do the right thing if we don't mess up
ref counts.

Wei.

> So I prefer following way I mentioned, suggestions?
> 
> >>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().
> 
> Thanks
> Annie

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