CVE-2025-40300
Description
In the Linux kernel, the following vulnerability has been resolved:
x86/vmscape: Add conditional IBPB mitigation
VMSCAPE is a vulnerability that exploits insufficient branch predictor isolation between a guest and a userspace hypervisor (like QEMU). Existing mitigations already protect kernel/KVM from a malicious guest. Userspace can additionally be protected by flushing the branch predictors after a VMexit.
Since it is the userspace that consumes the poisoned branch predictors, conditionally issue an IBPB after a VMexit and before returning to userspace. Workloads that frequently switch between hypervisor and userspace will incur the most overhead from the new IBPB.
This new IBPB is not integrated with the existing IBPB sites. For instance, a task can use the existing speculation control prctl() to get an IBPB at context switch time. With this implementation, the IBPB is doubled up: one at context switch and another before running userspace.
The intent is to integrate and optimize these cases post-embargo.
[ dhansen: elaborate on suboptimal IBPB solution ]
AI Insight
LLM-synthesized narrative grounded in this CVE's description and references.
CVE-2025-40300 is a Linux kernel vulnerability allowing branch predictor state leakage from guest to userspace hypervisor, mitigated by conditional IBPB after VMexit.
Vulnerability
CVE-2025-40300 (VMSCAPE) is a Linux kernel vulnerability that exploits insufficient branch predictor isolation between a guest virtual machine and a userspace hypervisor such as QEMU. While existing mitigations protect the kernel and KVM from malicious guests, the userspace hypervisor remains exposed to branch predictor state poisoning [2].
Exploitation
An attacker with guest access can manipulate the branch predictor to influence the userspace hypervisor's control flow, potentially leaking sensitive information. The fix introduces a conditional IBPB (Indirect Branch Prediction Barrier) issued after VMexit and before returning to userspace, preventing the predictor state from being consumed by the hypervisor. This new IBPB is not integrated with existing IBPB sites, which may cause redundant barriers in some scenarios [description].
Impact
Successful exploitation allows a malicious guest to leak data from the hosting userspace hypervisor, breaking the fundamental isolation guarantee in virtualization environments [2].
Mitigation
The patch is included in the Linux kernel as of the disclosed date (September 2025). Distributions such as Ubuntu have released updates (USN-7860-1) [2]. Xen hypervisor is not affected by this specific vulnerability [4].
AI Insight generated on May 19, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.
Affected products
1- Linux/Linuxv5Range: 4.16
Patches
0No patches discovered yet.
Vulnerability mechanics
AI mechanics synthesis has not run for this CVE yet.
References
21- git.kernel.org/stable/c/15006289e5c38b2a830e1fba221977a27598176cnvdPatch
- git.kernel.org/stable/c/2f4f2f8f860cb4c3336a7435ebe8dcfded0c9c6envdPatch
- git.kernel.org/stable/c/2f8f173413f1cbf52660d04df92d0069c4306d25nvdPatch
- git.kernel.org/stable/c/34e5667041050711a947e260fc9ebebe08bddee5nvdPatch
- git.kernel.org/stable/c/459274c77b37ac63b78c928b4b4e748d1f9d05c8nvdPatch
- git.kernel.org/stable/c/510603f504796c3535f67f55fb0b124a303b44c8nvdPatch
- git.kernel.org/stable/c/893387c18612bb452336a5881da0d015a7e8f4a2nvdPatch
- git.kernel.org/stable/c/9c23a90648e831d611152ac08dbcd1283d405e7fnvdPatch
- git.kernel.org/stable/c/ac60717f9a8d21c58617d0b34274babf24135835nvdPatch
- git.kernel.org/stable/c/c08192b5d6730a914dee6175bc71092ee6a65f14nvdPatch
- git.kernel.org/stable/c/d5490dfa35427a2967e00a4c7a1b95fdbc8ede34nvdPatch
- git.kernel.org/stable/c/d7ddc93392e4a7ffcccc86edf6ef3e64c778db52nvdPatch
- git.kernel.org/stable/c/f866eef8d1c65504d30923c3f14082ad294d0e6dnvdPatch
- lists.debian.org/debian-lts-announce/2025/10/msg00007.htmlnvdThird Party Advisory
- lists.debian.org/debian-lts-announce/2025/10/msg00008.htmlnvdThird Party Advisory
- www.openwall.com/lists/oss-security/2025/11/14/3nvdMailing List
- www.openwall.com/lists/oss-security/2025/11/14/4nvdMailing List
- www.openwall.com/lists/oss-security/2025/11/14/6nvdMailing List
- www.openwall.com/lists/oss-security/2025/11/17/2nvdMailing List
- www.openwall.com/lists/oss-security/2025/11/17/3nvdMailing List
- cert-portal.siemens.com/productcert/html/ssa-032379.htmlnvd
News mentions
1- Siemens SIMATICCISA ICS Advisories