Guests can trigger NIC interface reset/abort/crash via netback

ISSUE DESCRIPTION

It is possible for a guest to trigger a NIC interface reset/abort/crash in a Linux based network backend by sending certain kinds of packets.
It appears to be an (unwritten?) assumption in the rest of the Linux network stack that packet protocol headers are all contained within the linear section of the SKB and some NICs behave badly if this is not the case.
This has been reported to occur with Cisco (enic) and Broadcom NetXtrem II BCM5780 (bnx2x) though it may be an issue with other NICs/drivers as well.
In case the frontend is sending requests with split headers, netback will forward those violating above mentioned assumption to the networking core, resulting in said misbehavior.

IMPACT

An unprivileged guest can cause network Denial of Service (DoS) of the host by sending network packets to the backend causing the related physical NIC to reset, abort, or crash.
Data corruption or privilege escalation seem unlikely but have not been ruled out.

VULNERABLE SYSTEMS

All systems using a Linux based network backend with kernel 3.19 and newer are vulnerable. Systems using other network backends are not known to be vulnerable.
Systems using Cisco (enic driver) and Broadcom NetXtrem II BCM5780 (bnx2x driver) NICs for guest network access are known to be vulnerable. Systems using other NICs for guest network access cannot be ruled out to be vulnerable.