[XSA-406] Arm guests can cause Dom0 DoS via PV devices

ISSUE DESCRIPTION

When mapping pages of guests on Arm, dom0 is using an rbtree to keep
track of the foreign mappings.

Updating of that rbtree is not always done completely with the related
lock held, resulting in a small race window, which can be used by
unprivileged guests via PV devices to cause inconsistencies of the
rbtree. These inconsistencies can lead to Denial of Service (DoS) of
dom0, e.g. by causing crashes or the inability to perform further
mappings of other guests' memory pages.

IMPACT

A guest performing multiple I/Os of PV devices in parallel can cause
DoS of dom0 and thus of the complete host.

VULNERABLE SYSTEMS

Only Arm systems (32-bit and 64-bit) are vulnerable. Dom0 Linux versions
3.13 - 5.18 are vulnerable.

X86 systems are not vulnerable.