[XSA-448] Linux: netback processing of zero-length transmit fragment

ISSUE DESCRIPTION

Transmit requests in Xen's virtual network protocol can consist of
multiple parts. While not really useful, except for the initial part
any of them may be of zero length, i.e. carry no data at all. Besides a
certain initial portion of the to be transferred data, these parts are
directly translated into what Linux calls SKB fragments. Such converted
request parts can, when for a particular SKB they are all of length
zero, lead to a de-reference of NULL in core networking code.

IMPACT

An unprivileged guest can cause Denial of Service (DoS) of the host by
sending network packets to the backend, causing the backend to crash.

Data corruption or privilege escalation have not been ruled out.

VULNERABLE SYSTEMS

All systems using a Linux based network backend with kernel 4.14 and
newer are vulnerable. Earlier versions may also be vulnerable. Systems
using other network backends are not known to be vulnerable.