CVE-2020-25597

A logic flaw was found in the handling of event channel operations in Xen. Operations such as resetting all event channels may involve decreasing one of the bounds checked when determining validity. This flaw allows an unprivileged guest to crash Xen, leading to a denial of service (DoS) for the entire system. The highest threat from this vulnerability is to system availability.

Mitigation

The issue can be avoided by reducing the number of event channels available to the guest to no more than 1023. For example, setting max_event_channels=1023 in the xl domain configuration, or deleting any existing setting (since 1023 is the default for xl/libxl).

For ARM systems, any limit no more than 4095 is safe. For 64-bit x86 PV guests, any limit no more than 4095 is likewise safe if the host configuration prevents the guest administrator from substituting and running a 32-bit kernel (and thereby putting the guest into 32-bit PV mode).