CVE-2020-25597

An issue was discovered in Xen through 4.14.x. There is mishandling of the
constraint that once-valid event channels may not turn invalid. Logic in
the handling of event channel operations in Xen assumes that an event
channel, once valid, will not become invalid over the life time of a guest.
However, operations like the resetting of all event channels may involve
decreasing one of the bounds checked when determining validity. This may
lead to bug checks triggering, crashing the host. An unprivileged guest may
be able to crash Xen, leading to a Denial of Service (DoS) for the entire
system. All Xen versions from 4.4 onwards are vulnerable. Xen versions 4.3
and earlier are not vulnerable. Only systems with untrusted guests
permitted to create more than the default number of event channels are
vulnerable. This number depends on the architecture and type of guest. For
32-bit x86 PV guests, this is 1023; for 64-bit x86 PV guests, and for all
ARM guests, this number is 4095. Systems where untrusted guests are limited
to fewer than this number are not vulnerable. Note that xl and libxl limit
max_event_channels to 1023 by default, so systems using exclusively xl,
libvirt+libxl, or their own toolstack based on libxl, and not explicitly
setting max_event_channels, are not vulnerable.

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