xen-netback: don't disconnect frontend when seeing oversize packet

Some frontend drivers are sending packets > 64 KiB in length. This length
overflows the length field in the first slot making the following slots have
an invalid length.

Turn this error back into a non-fatal error by dropping the packet. To avoid
having the following slots having fatal errors, consume all slots in the
packet.

This does not reopen the security hole in XSA-39 as if the packet as an
invalid number of slots it will still hit fatal error case.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 16 insertions, 6 deletions

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index d9292c59789b..a2865f17c667 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -975,12 +975,22 @@ 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,
-                                   "Invalid tx request, slot size %u > remaining size %u\n",
-                                   txp->size, first->size);
-                        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_err && txp->size > first->size) {
+                        if (net_ratelimit())
+                                netdev_dbg(vif->dev,
+                                           "Invalid tx request, slot size %u > remaining size %u\n",
+                                           txp->size, first->size);
+                        drop_err = -EIO;
                 }
 
                 first->size -= txp->size;