CVE-2015-5307

What it actually does

Guest-triggered flood of #AC exceptions in KVM (svm.c/vmx.c) or Xen causes host kernel panic or hang, resulting in complete denial of service for the hypervisor and all hosted guests.

Why it matters to the business

Unexpected host-level outages that take down every virtual machine on the affected server, leading to service disruption, data inaccessibility, and unplanned recovery time across virtualized environments.

How an attack unfolds

An attacker who already controls a guest virtual machine executes code that repeatedly triggers alignment-check exceptions; the hypervisor fails to rate-limit or safely handle the exception storm, causing the host kernel to panic or hang.

How to fix it

Upgrade the Linux kernel to a version containing the fix for the exception-handling paths in svm.c and vmx.c, or upgrade Xen to a release after 4.6.x that properly bounds guest-induced exceptions; apply vendor-supplied hypervisor and kernel updates as soon as they become available.

How to confirm the fix worked

Confirm the running kernel or Xen version contains the upstream patches that limit or safely handle repeated #AC exceptions; test by attempting controlled exception generation in a lab guest and verifying the host remains stable.

Typical attack path

1. Obtain code execution inside a guest VM
2. Run a loop or tool that forces repeated #AC exceptions
3. Observe host kernel panic or hang from the exception flood
4. All other guests on the same host become unavailable

How to detect it

Review hypervisor and kernel logs for repeated alignment-check exception messages immediately preceding a panic or hang; correlate guest activity with host instability events.

• kernel: alignment check
• #AC exception flood
• general protection fault followed by host panic in kvm or xen

What your security team should do

• Prioritize patching of all hypervisor hosts running affected KVM or Xen versions
• Isolate or limit untrusted guest workloads until patches are applied
• Monitor hypervisor logs for unusual volumes of alignment-check exceptions