Security controls bypass or absence In xen

Description

In the context switch logic Xen attempts to skip an IBPB in the case of a vCPU returning to a CPU on which it was the previous vCPU to run. While safe for Xen's isolation between vCPUs, this prevents the guest kernel correctly isolating between tasks. Consider:

    vCPU runs on CPU A, running task 1.

    vCPU moves to CPU B, idle gets scheduled on A. Xen skips IBPB.

    On CPU B, guest kernel switches from task 1 to 2, issuing IBPB.

    vCPU moves back to CPU A. Xen skips IBPB again.

Now, task 2 is running on CPU A with task 1's training still in the BTB.

Mitigation

Update Impact

Minimal update. May introduce new vulnerabilities or breaking changes.

Ecosystem
Package
Affected version
Patched versions