Linux: special config may crash when trying to map foreign pages

ISSUE DESCRIPTION

With CONFIG_XEN_BALLOON_MEMORY_HOTPLUG disabled and CONFIG_XEN_UNPOPULATED_ALLOC enabled the Linux kernel will use guest physical addresses allocated via the ZONE_DEVICE functionality for mapping foreign guest’s pages.
This will result in problems, as the p2m list will only cover the initial memory size of the domain plus some padding at the end. Most ZONE_DEVICE allocated addresses will be outside the p2m range and thus a mapping can’t be established with those memory addresses, resulting in a crash.
The attack involves doing I/O requiring large amounts of data to be mapped by the Dom0 or driver domain. The amount of data needed to result in a crash can vary depending on the memory layout of the affected Dom0 or driver domain.

IMPACT

A Dom0 or driver domain based on a Linux kernel (configured as described above) can be crashed by a malicious guest administrator, or possibly malicious unprivileged guest processes.

VULNERABLE SYSTEMS

Only x86 paravirtualized (PV) Dom0 or driver domains are affected.
Only Linux kernels configured with CONFIG_XEN_UNPOPULATED_ALLOC and without CONFIG_XEN_BALLOON_MEMORY_HOTPLUG are vulnerable. Only kernels from kernel version 5.9 onwards are affected.
CONFIG_XEN_BALLOON_MEMORY_HOTPLUG is enabled by default in upstream Linux when Xen support is enabled, so kernels using upstream default Kconfig are not affected. Most distribution kernels supporting Xen dom0 use are likewise not vulnerable.
Arm systems or x86 PVH or x86 HVM driver domains are not affected.