fix(prepare): disable KVM nested virtualization (CVE-2026-53359)#60
fix(prepare): disable KVM nested virtualization (CVE-2026-53359)#60Andrei Kvapil (kvaps) wants to merge 1 commit into
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Ship an /etc/modprobe.d/cozystack-kvm-nested.conf drop-in (options kvm_intel/kvm_amd nested=0) on Ubuntu, RHEL and SUSE host-prep to mitigate CVE-2026-53359 (Januscape), a KVM guest-to-host escape reachable only when nested virtualization is exposed to the guest. Gated by cozystack_disable_kvm_nested (default true); opt out to re-enable nested virt. Signed-off-by: Andrei Kvapil <kvapss@gmail.com> Assisted-By: Claude <noreply@anthropic.com>
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Code Review
This pull request introduces a mitigation for CVE-2026-53359 by disabling KVM nested virtualization across RHEL, SUSE, and Ubuntu environments. It adds Ansible tasks to write a modprobe configuration file (/etc/modprobe.d/cozystack-kvm-nested.conf) that sets nested=0 for both Intel and AMD KVM modules, and includes a cleanup task to remove the file if the mitigation is disabled. There are no review comments, and I have no additional feedback to provide.
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Andrei Kvapil (kvaps)
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Summary
Disables KVM nested virtualization on generic (non-Talos) Ubuntu/RHEL/SUSE hosts prepared for Cozystack, to mitigate CVE-2026-53359 ("Januscape"), a guest-to-host escape in the Linux KVM/x86 shadow MMU (a use-after-free in
kvm_mmu_get_child_sp()from shadow-page role confusion).The vulnerable code path is only reachable when a guest is allowed to use nested virtualization; disabling it on the host removes the attack surface regardless of the node kernel patch level. Fixed upstream in
81ccda30b4e8/ stable 6.12.95, 6.6.144, 6.1.177, 6.18.38, 7.1.3 (2026-07-04).Changes
/etc/modprobe.d/cozystack-kvm-nested.confdrop-in (options kvm_intel nested=0/options kvm_amd nested=0) inexamples/{ubuntu,rhel,suse}/prepare-*.yml, written before thekvm_*module is loaded (mirrors the existing DRBD modprobe.d ordering). Both options are set so one drop-in covers Intel and AMD.cozystack_disable_kvm_nestedvariable (defaulttrue); a paired cleanup task removes the drop-in when opted out or when KubeVirt is disabled.kvm_intel/kvm_amdis already loaded, a reboot (or module reload) is required for the change to take effect.Part of a coordinated set of PRs (see Related PRs below).
Test plan
ansible-lintpassesansible-test sanitypassesVerified locally:
ansible-playbook --syntax-checkpasses on all three playbooks.ansible-lint/ live / idempotency not yet run — left for CI / reviewer.Related PRs
Coordinated CVE-2026-53359 (Januscape) nested-virtualization mitigation across Cozystack repos:
cozystack/genericpresets (canonical source)