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FedoraOperating system · Fedoraproject

CVE-2021-28704

HIGH · 8.8 CVSS v3.1 Published 2021-11-24
Fix available
A fix is available. Upgrade to after 4.15.1 or later.
See remediation →
90/100
Remediation priority · Urgent
Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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).

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis

A detailed technical summary for this CVE is being prepared.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FedoraOperating system
Affected:= 34= 35
Debian LinuxOperating system
Affected:= 9.0
XenOperating system
Affected:>= 4.7.0, <= 4.15.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

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

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to a release after 4.15.1
Vendor patch xenbits.xenproject.org →
Recommended fix Moderate confidence

Xen 4.15.2 or later

  1. Identify current Xen version using 'xl version' or 'xm version'
  2. Schedule maintenance window and gracefully shut down all running VMs
  3. Back up Xen configuration files in /etc/xen/
  4. Upgrade Xen hypervisor package to version 4.15.2 or later via distribution package manager (e.g., 'dnf upgrade' for Fedora, 'apt-get upgrade' for Debian)
  5. Verify Xen package version after upgrade
  6. Reboot the host to load the new hypervisor
  7. Confirm hypervisor version matches the fixed release (greater than 4.15.1)
  8. Start VMs and verify normal operation
Caveat Hypervisor upgrade requires downtime; all VMs must be shut down before rebooting the host. Test VM configurations and storage accessibility after upgrade in a staging environment first.

Generated from the published advisory — verify against the referenced sources before acting.

We can identify the exact fixed release, upgrade, and verify it in staging — typical engagement from $1,950. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-28704 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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