CVE-2021-28704: PoD operations on misaligned GFNs T[his CNA information record relates to multiple CVEs; the text explains which aspects/vulnerabilities correspond to...

Description

PoD operations on misaligned GFNs T[his CNA information record relates to multiple CVEs; the text explains which aspects/vulnerabilities correspond to which CVE.] x86 HVM and PVH guests may be started in populate-on-demand (PoD) mode, to provide a way for them to later easily have more memory assigned. Guests are permitted to control certain P2M aspects of individual pages via hypercalls. These hypercalls may act on ranges of pages specified via page orders (resulting in a power-of-2 number of pages). The implementation of some of these hypercalls for PoD does not enforce the base page frame number to be suitably aligned for the specified order, yet some code involved in PoD handling actually makes such an assumption. These operations are XENMEM_decrease_reservation (CVE-2021-28704) and XENMEM_populate_physmap (CVE-2021-28707), the latter usable only by domains controlling the guest, i.e. a de-privileged qemu or a stub domain. (Patch 1, combining the fix to both these two issues.) In addition handling of XENMEM_decrease_reservation can also trigger a host crash when the specified page order is neither 4k nor 2M nor 1G (CVE-2021-28708, patch 2).

Metrics

CVSS 3.1

8.8/10

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H

EPSS Probability

0.33%

24.5th percentile

Probability of exploitation in the next 30 days. Learn more

Affected Software

Vendor	Product	Versions
Xen	Xen	>= 4.7.0, <= 4.15.1
Fedoraproject	Fedora	34
Fedoraproject	Fedora	35
Debian	Debian Linux	9.0

References

Timeline

Published: Nov 24, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-28704?

PoD operations on misaligned GFNs T[his CNA information record relates to multiple CVEs; the text explains which aspects/vulnerabilities correspond to which CVE.] x86 HVM and PVH guests may be started in populate-on-demand (PoD) mode, to provide a way for them to later easily have more memory assigned. Guests are permitted to control certain P2M aspects of individual pages via hypercalls. These hypercalls may act on ranges of pages specified via page orders (resulting in a power-of-2 number of pages). The implementation of some of these hypercalls for PoD does not enforce the base page frame number to be suitably aligned for the specified order, yet some code involved in PoD handling actually makes such an assumption. These operations are XENMEM_decrease_reservation (CVE-2021-28704) and XENMEM_populate_physmap (CVE-2021-28707), the latter usable only by domains controlling the guest, i.e. a de-privileged qemu or a stub domain. (Patch 1, combining the fix to both these two issues.) In addition handling of XENMEM_decrease_reservation can also trigger a host crash when the specified page order is neither 4k nor 2M nor 1G (CVE-2021-28708, patch 2).

How severe is CVE-2021-28704?

CVE-2021-28704 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 0.33% probability of exploitation in the next 30 days.

How do I fix CVE-2021-28704?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.