x86: incomplete IBPB for vCPU isolation_CVE-2026-23553

2.9 / 10

LOW

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

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:

1) vCPU runs on CPU A, running task 1.
2) vCPU moves to CPU B, idle gets scheduled on A. Xen skips IBPB.
3) On CPU B, guest kernel switches from task 1 to 2, issuing IBPB.
4) 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.

Basic Information

ID CVE-2026-23553

Source XEN

Published Jan 28, 2026 at 15:33

Modified Jan 28, 2026 at 16:41

Affected Product

Vendor Xen

Product Xen

Version consult Xen advisory XSA-479

CWE Classification

CWE-665 CWE-693

References

xenbits.xenproject.org /xsa/advisory-479.html

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    "published": "2026-01-28T15:33:44.782Z",
    "modified": "2026-01-28T16:41:14.803Z",
    "type": "cve",
    "title": "x86: incomplete IBPB for vCPU isolation",
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