[Xen-devel] [PATCH 6/6] xen-netback: don't disconnect frontend when seeing oversize frame

[Wei Liu <wei.liu2@xxxxxxxxxx>]

Some buggy frontends may generate frames larger than 64 KiB. We should
aggresively consume all slots and drop the packet instead of disconnecting the
frontend.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
---
 drivers/net/xen-netback/netback.c |   32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index a634dc5..1971623 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -924,10 +924,12 @@ 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,
+                               RING_IDX first_idx)
 {
        RING_IDX cons = vif->tx.req_cons;
        int slots = 0;
+       bool drop = false;
 
        if (!(first->flags & XEN_NETTXF_more_data))
                return 0;
@@ -947,10 +949,21 @@ static int netbk_count_requests(struct xenvif *vif,
 
                memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + slots),
                       sizeof(*txp));
-               if (txp->size > first->size) {
-                       netdev_err(vif->dev, "Packet 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 slots will
+                * appear to be larger than the frame.
+                *
+                * This cannot be fatal error as there are buggy
+                * frontends that do this.
+                *
+                * Consume all slots and drop the packet.
+                */
+               if (!drop && txp->size > first->size) {
+                       if (net_ratelimit())
+                               netdev_dbg(vif->dev,
+                                          "Packet is bigger than frame.\n");
+                       drop = true;
                }
 
                first->size -= txp->size;
@@ -963,6 +976,12 @@ static int netbk_count_requests(struct xenvif *vif,
                        return -EINVAL;
                }
        } while ((txp++)->flags & XEN_NETTXF_more_data);
+
+       if (drop) {
+               netbk_tx_err(vif, first, first_idx + slots);
+               return -EIO;
+       }
+
        return slots;
 }
 
@@ -1429,7 +1448,8 @@ 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;
 
-- 
1.7.10.4


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