Incorrect error handling in event channel port allocation

ISSUE DESCRIPTION

The allocation of an event channel port may fail for multiple reasons: 1) Port is already in use 2) The memory allocation failed 3) The port we try to allocate is higher than what is supported by the ABI (e.g 2L or FIFO) used by the guest or the limit set by an administrator (‘max_event_channels’ in xl cfg).
Due to the missing error checks, only 1) will be considered as an error. All the other cases will provide a “valid” port and will result to a crash when trying to access the event channel.

IMPACT

When the administrator configured a guest to allow more than 1023 event channels, that guest may be able to crash the host.
When Xen is out-of-memory, allocation of new event channels will result in crashing the host rather than reporting an error.

VULNERABLE SYSTEMS

Xen versions 4.10 and later are affected. (The special Xen 4.8 “Comet” branch for XSA-254 contains changes similar to those which led to this vulnerability; so it is likely to be affected, but - like mainline Xen 4.8 - that branch is longer security-supported.)
Older Xen versions are unaffected.
All architectures are affected.
The default configuration, when guests are created with xl/libxl, is not vulnerable, because of the default event channel limit (see Mitigation, below).