Re: [Xen-devel] [PATCH 2/4] xen-netfront: drop skb when skb->len > 65535

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 18/03/13 13:48, Ian Campbell wrote:
> On Mon, 2013-03-18 at 13:46 +0000, David Vrabel wrote:
>> On 18/03/13 10:35, Wei Liu wrote:
>>> The `size' field of Xen network wire format is uint16_t, anything bigger 
>>> than
>>> 65535 will cause overflow.
>>
>> The backend needs to be able to handle these bad packets without
>> disconnecting the VIF -- we can't fix all the frontend drivers.
> 
> Agreed, although that doesn't imply that we shouldn't fix the frontend
> where we can -- such as upstream as Wei does here.

Yes, frontends should be fixed where possible.

This is what I came up with for the backend.  I don't have time to look
into it further but, Wei, feel free to use it as a starting point.

David

diff --git a/drivers/net/xen-netback/netback.c
b/drivers/net/xen-netback/netback.c
index cd49ba9..18e2671 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -899,10 +899,11 @@ static void netbk_fatal_tx_err(struct xenvif *vif)
 static int netbk_count_requests(struct xenvif *vif,
                                struct xen_netif_tx_request *first,
                                struct xen_netif_tx_request *txp,
-                               int work_to_do)
+                               int work_to_do, int idx)
 {
        RING_IDX cons = vif->tx.req_cons;
        int frags = 0;
+       bool drop = false;

        if (!(first->flags & XEN_NETTXF_more_data))
                return 0;
@@ -922,10 +923,20 @@ static int netbk_count_requests(struct xenvif *vif,

                memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + frags),
                       sizeof(*txp));
-               if (txp->size > first->size) {
-                       netdev_err(vif->dev, "Frag is bigger than frame.\n");
-                       netbk_fatal_tx_err(vif);
-                       return -EIO;
+
+               /*
+                * If the guest submitted a frame >= 64 KiB then
+                * first->size overflowed and following frags will
+                * appear to be larger than the frame.
+                *
+                * This cannot be a fatal error as there are buggy
+                * frontends that do this.
+                *
+                * Consume all the frags and drop the packet.
+                */
+               if (!drop && txp->size > first->size) {
+                       netdev_dbg(vif->dev, "Frag is bigger than frame.\n");
+                       drop = true;
                }

                first->size -= txp->size;
@@ -938,6 +949,12 @@ static int netbk_count_requests(struct xenvif *vif,
                        return -EINVAL;
                }
        } while ((txp++)->flags & XEN_NETTXF_more_data);
+
+       if (drop) {
+               netbk_tx_err(vif, txp, idx + frags);
+               return -EIO;
+       }
+
        return frags;
 }

@@ -1327,7 +1344,7 @@ static unsigned xen_netbk_tx_build_gops(struct
xen_netbk *netbk)
                                continue;
                }

-               ret = netbk_count_requests(vif, &txreq, txfrags, work_to_do);
+               ret = netbk_count_requests(vif, &txreq, txfrags, work_to_do, 
idx);
                if (unlikely(ret < 0))
                        continue;


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