CVE-2020-29566

An issue was discovered in Xen through 4.14.x. When they require assistance
from the device model, x86 HVM guests must be temporarily de-scheduled. The
device model will signal Xen when it has completed its operation, via an
event channel, so that the relevant vCPU is rescheduled. If the device
model were to signal Xen without having actually completed the operation,
the de-schedule / re-schedule cycle would repeat. If, in addition, Xen is
resignalled very quickly, the re-schedule may occur before the de-schedule
was fully complete, triggering a shortcut. This potentially repeating
process uses ordinary recursive function calls, and thus could result in a
stack overflow. A malicious or buggy stubdomain serving a HVM guest can
cause Xen to crash, resulting in a Denial of Service (DoS) to the entire
host. Only x86 systems are affected. Arm systems are not affected. Only x86
stubdomains serving HVM guests can exploit the vulnerability.

Notes