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 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.

Wei.

> 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().
> 
> Thanks
> Annie

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel