RE: [Xen-devel] [RFC PATCH 3/3] xen/netback: Fix grant copy across page boundary with KASAN

["Durrant, Paul" <pdurrant@amazon.com>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@lists.xenproject.org> On Behalf Of
> Sergey Dyasli
> Sent: 17 December 2019 14:08
> To: xen-devel@lists.xen.org; kasan-dev@googlegroups.com; linux-
> kernel@vger.kernel.org
> Cc: Juergen Gross <jgross@suse.com>; Sergey Dyasli
> <sergey.dyasli@citrix.com>; Stefano Stabellini <sstabellini@kernel.org>;
> George Dunlap <george.dunlap@citrix.com>; Ross Lagerwall
> <ross.lagerwall@citrix.com>; Alexander Potapenko <glider@google.com>;
> Andrey Ryabinin <aryabinin@virtuozzo.com>; Boris Ostrovsky
> <boris.ostrovsky@oracle.com>; Dmitry Vyukov <dvyukov@google.com>
> Subject: [Xen-devel] [RFC PATCH 3/3] xen/netback: Fix grant copy across
> page boundary with KASAN
> 
> From: Ross Lagerwall <ross.lagerwall@citrix.com>
> 
> When KASAN (or SLUB_DEBUG) is turned on, the normal expectation that
> allocations are aligned to the next power of 2 of the size does not
> hold. Therefore, handle grant copies that cross page boundaries.
> 
> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
> Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>

Would have been nice to cc netback maintainers...

> ---
>  drivers/net/xen-netback/common.h  |  2 +-
>  drivers/net/xen-netback/netback.c | 55 ++++++++++++++++++++++++-------
>  2 files changed, 45 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-
> netback/common.h
> index 05847eb91a1b..e57684415edd 100644
> --- a/drivers/net/xen-netback/common.h
> +++ b/drivers/net/xen-netback/common.h
> @@ -155,7 +155,7 @@ struct xenvif_queue { /* Per-queue data for xenvif */
>  	struct pending_tx_info pending_tx_info[MAX_PENDING_REQS];
>  	grant_handle_t grant_tx_handle[MAX_PENDING_REQS];
> 
> -	struct gnttab_copy tx_copy_ops[MAX_PENDING_REQS];
> +	struct gnttab_copy tx_copy_ops[MAX_PENDING_REQS * 2];
>  	struct gnttab_map_grant_ref tx_map_ops[MAX_PENDING_REQS];
>  	struct gnttab_unmap_grant_ref tx_unmap_ops[MAX_PENDING_REQS];
>  	/* passed to gnttab_[un]map_refs with pages under (un)mapping */
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> index 0020b2e8c279..1541b6e0cc62 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -320,6 +320,7 @@ static int xenvif_count_requests(struct xenvif_queue
> *queue,
> 
>  struct xenvif_tx_cb {
>  	u16 pending_idx;
> +	u8 copies;
>  };

I know we're a way off the limit (48 bytes) but I wonder if we ought to have a compile time check here that we're not overflowing skb->cb.

> 
>  #define XENVIF_TX_CB(skb) ((struct xenvif_tx_cb *)(skb)->cb)
> @@ -439,6 +440,7 @@ static int xenvif_tx_check_gop(struct xenvif_queue
> *queue,
>  {
>  	struct gnttab_map_grant_ref *gop_map = *gopp_map;
>  	u16 pending_idx = XENVIF_TX_CB(skb)->pending_idx;
> +	u8 copies = XENVIF_TX_CB(skb)->copies;
>  	/* This always points to the shinfo of the skb being checked, which
>  	 * could be either the first or the one on the frag_list
>  	 */
> @@ -450,23 +452,27 @@ static int xenvif_tx_check_gop(struct xenvif_queue
> *queue,
>  	int nr_frags = shinfo->nr_frags;
>  	const bool sharedslot = nr_frags &&
>  				frag_get_pending_idx(&shinfo->frags[0]) ==
> pending_idx;
> -	int i, err;
> +	int i, err = 0;
> 
> -	/* Check status of header. */
> -	err = (*gopp_copy)->status;
> -	if (unlikely(err)) {
> -		if (net_ratelimit())
> -			netdev_dbg(queue->vif->dev,
> +	while (copies) {
> +		/* Check status of header. */
> +		int newerr = (*gopp_copy)->status;
> +		if (unlikely(newerr)) {
> +			if (net_ratelimit())
> +				netdev_dbg(queue->vif->dev,
>  				   "Grant copy of header failed! status: %d
> pending_idx: %u ref: %u\n",
>  				   (*gopp_copy)->status,
>  				   pending_idx,
>  				   (*gopp_copy)->source.u.ref);
> -		/* The first frag might still have this slot mapped */
> -		if (!sharedslot)
> -			xenvif_idx_release(queue, pending_idx,
> -					   XEN_NETIF_RSP_ERROR);
> +			/* The first frag might still have this slot mapped */
> +			if (!sharedslot && !err)
> +				xenvif_idx_release(queue, pending_idx,
> +						   XEN_NETIF_RSP_ERROR);

Can't this be done after the loop, if there is an accumulated err? I think it would make the code slightly neater.

> +			err = newerr;
> +		}
> +		(*gopp_copy)++;
> +		copies--;
>  	}
> -	(*gopp_copy)++;
> 
>  check_frags:
>  	for (i = 0; i < nr_frags; i++, gop_map++) {
> @@ -910,6 +916,7 @@ static void xenvif_tx_build_gops(struct xenvif_queue
> *queue,
>  			xenvif_tx_err(queue, &txreq, extra_count, idx);
>  			break;
>  		}
> +		XENVIF_TX_CB(skb)->copies = 0;
> 
>  		skb_shinfo(skb)->nr_frags = ret;
>  		if (data_len < txreq.size)
> @@ -933,6 +940,7 @@ static void xenvif_tx_build_gops(struct xenvif_queue
> *queue,
>  						   "Can't allocate the frag_list
> skb.\n");
>  				break;
>  			}
> +			XENVIF_TX_CB(nskb)->copies = 0;
>  		}
> 
>  		if (extras[XEN_NETIF_EXTRA_TYPE_GSO - 1].type) {
> @@ -990,6 +998,31 @@ static void xenvif_tx_build_gops(struct xenvif_queue
> *queue,
> 
>  		queue->tx_copy_ops[*copy_ops].len = data_len;

If offset_in_page(skb->data)+ data_len can exceed XEN_PAGE_SIZE, does this not need to be truncated?

  Paul

>  		queue->tx_copy_ops[*copy_ops].flags = GNTCOPY_source_gref;
> +		XENVIF_TX_CB(skb)->copies++;
> +
> +		if (offset_in_page(skb->data) + data_len > XEN_PAGE_SIZE) {
> +			unsigned int extra_len = offset_in_page(skb->data) +
> +					     data_len - XEN_PAGE_SIZE;
> +
> +			queue->tx_copy_ops[*copy_ops].len -= extra_len;
> +			(*copy_ops)++;
> +
> +			queue->tx_copy_ops[*copy_ops].source.u.ref = txreq.gref;
> +			queue->tx_copy_ops[*copy_ops].source.domid =
> +				queue->vif->domid;
> +			queue->tx_copy_ops[*copy_ops].source.offset =
> +				txreq.offset + data_len - extra_len;
> +
> +			queue->tx_copy_ops[*copy_ops].dest.u.gmfn =
> +				virt_to_gfn(skb->data + data_len - extra_len);
> +			queue->tx_copy_ops[*copy_ops].dest.domid = DOMID_SELF;
> +			queue->tx_copy_ops[*copy_ops].dest.offset = 0;
> +
> +			queue->tx_copy_ops[*copy_ops].len = extra_len;
> +			queue->tx_copy_ops[*copy_ops].flags =
> GNTCOPY_source_gref;
> +
> +			XENVIF_TX_CB(skb)->copies++;
> +		}
> 
>  		(*copy_ops)++;
> 
> --
> 2.17.1
> 
> 
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