[Xen-devel] [PATCH V4 7/7] xen-netback: don't disconnect frontend when seeing oversize packet

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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@xxxxxxxxxx>
Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
---
 drivers/net/xen-netback/netback.c |   22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 00ae841..e21c15e 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -993,12 +993,22 @@ 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;
-- 
1.7.10.4


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