domctl lock open to abuse

[This CNA information record relates to multiple CVEs; the
text explains which aspects/vulnerabilities correspond to which CVE.]

To create and manage guests, domctl operations are used by the control
domain, a possible Xenstore domain, or by a domain controlling a
particular guest. Some of these operations may not be executed in
parallel, so a system-wide lock is used. The way that lock is acquired
is, however, not providing any fairness. This is CVE-2026-42489.

Furthermore, with XSM/Flask in use, the lock acquire will, for some
operations, occur ahead of any permission checking. This is
CVE-2026-42490.