Linux netback crash trying to disable due to malformed packet

ISSUE DESCRIPTION

When Linux’s netback sees a malformed packet, it tries to disable the interface which serves the misbehaving frontend.
This involves taking a mutex, which might sleep. But in recent versions of Linux the guest transmit path is handled by NAPI in softirq context, where sleeping is not allowed. The end result is that the backend domain (often, Dom0) crashes with “scheduling while atomic”.

IMPACT

Malicious guest administrators can cause denial of service. If driver domains are not in use, the impact is a host crash.

VULNERABLE SYSTEMS

This bug affects systems using Linux as the driver domain, including non-disaggregated systems using Linux as dom0.
Only versions of Linux whose netback uses NAPI are affected. In Linux mainline this is all versions of Linux containing git changeset b3f980bd82, which was introduced between Linux 3.11 and 3.12-rc1.
Systems using a different OS as dom0 (eg, NetBSD, Solaris) are not vulnerable.
Both x86 and ARM systems are affected.