CVE-2026-102703: arbitrary read/write via grant copy

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

A common access pattern is a batch of Grant Copy operations acting on adjacent address ranges in the same source or destination page. For performance reasons, Xen caches details internally to reuse already validated data. The conditions on when to re-validate the internal data were not correct in some cases:

- Frame numbers and grant references could wrongly be mixed up. This is CVE-2026-102703.

- Copying by frame number (rather than grant reference) is permitted only when operating on the caller's own memory. The check to enforce this restriction was misplaced. This is CVE-2026-102704.