CVE-2026-79609: x86: XPTI may break on huge-memory systems
XPTI (Xen Page Table Isolation) was introduced to mitigate Meltdown (CVE-2017-5754, one aspect of XSA-254). This involves copying 64-bit PV guest page table entries from guest-chosen root page tables to the Xen- chosen one which is in actual use by hardware. To read the guest root page table, the directmap is used. PV guests, however, have a directmap which may not cover all memory; in particular this is the case if any memory extends beyond the 5 TiB (CONFIGBIGMEM=y: 3.5 TiB) address boundary.
Affected Software
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
The issue concerns 64-bit PV guests using XPTI where guest memory extends beyond the directmap coverage limit. This can occur above 5 TiB of memory, or above 3.5 TiB when CONFIG_BIGMEM=y.
What configuration condition triggers the problem?
XPTI copies page-table entries from a guest-selected root page table using the guest directmap. The problem arises when the required guest memory is outside that directmap, which is possible on huge-memory PV guest configurations.