[XSA-441] Possible deadlock in Linux kernel event handling

ISSUE DESCRIPTION

Closing of an event channel in the Linux kernel can result in a deadlock.
This happens when the close is being performed in parallel to an unrelated
Xen console action and the handling of a Xen console interrupt in an
unprivileged guest.

The closing of an event channel is e.g. triggered by removal of a
paravirtual device on the other side. As this action will cause console
messages to be issued on the other side quite often, the chance of
triggering the deadlock is not neglectable.

Note that 32-bit Arm-guests are not affected, as the 32-bit Linux kernel
on Arm doesn't use queued-RW-locks, which are required to trigger the
issue (on Arm32 a waiting writer doesn't block further readers to get
the lock).

IMPACT

A (malicious) guest administrator could cause a denial of service (DoS)
in a backend domain (other than dom0) by disabling a paravirtualized
device.

A malicious backend could cause DoS in a guest running a Linux kernel by
disabling a paravirtualized device.

VULNERABLE SYSTEMS

All unprivileged guests running a Linux kernel of version 5.10 and later,
or with the fixes for XSA-332, are vulnerable.

All guest types are vulnerable.

Only x86- and 64-bit Arm-guests are vulnerable.

Arm-guests running in 32-bit mode are not vulnerable.

Guests not using paravirtualized drivers are not vulnerable.