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

From: David Vrabel <[email-masked]>
Date: Wed, 15 Jan 2014 11:20:49 +0000

On 09/01/14 22:48, Annie Li wrote:
> Current netfront only grants pages for grant copy, not for grant transfer, so
> remove corresponding transfer code and add receiving copy code in
> xennet_release_rx_bufs.

While netfront only supports a copying backend, I don't see anything
preventing the backend from retaining mappings to netfront's Rx buffers...

> Signed-off-by: Annie Li <[email-masked]>
> ---
>  drivers/net/xen-netfront.c |   60 ++-----------------------------------------
>  1 files changed, 3 insertions(+), 57 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index e59acb1..692589e 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -1134,78 +1134,24 @@ static void xennet_release_tx_bufs(struct netfront_info *np)
>  
>  static void xennet_release_rx_bufs(struct netfront_info *np)
>  {
[...]
> -		mfn = gnttab_end_foreign_transfer_ref(ref);
> +		gnttab_end_foreign_access_ref(ref, 0);

... the gnttab_end_foreign_access_ref() may then fail and...

>  		gnttab_release_grant_reference(&np->gref_rx_head, ref);
>  		np->grant_rx_ref[id] = GRANT_INVALID_REF;
[...]
> +		kfree_skb(skb);

... this could then potentially free pages that the backend still has
mapped.  If the pages are then reused, this would leak information to
the backend.

Since only a buggy backend would result in this, leaking the skbs and
grant refs would be acceptable here.  I would also print an error.

While checking blkfront for how it handles this, it also doesn't appear
to do the right thing either.

David