CVE-2025-40300

MEDIUM
Published Sep 11, 2025 Modified May 12, 2026

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 ]

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CVSS v3.1 Score

5.5
MEDIUM
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected Products

Vendor Product
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
linux linux_kernel
debian debian_linux

References

Advisories & Patches

https://git.kernel.org/stable/c/15006289e5c38b2a830e1fba221977a27598176c https://git.kernel.org/stable/c/2f4f2f8f860cb4c3336a7435ebe8dcfded0c9c6e https://git.kernel.org/stable/c/2f8f173413f1cbf52660d04df92d0069c4306d25 https://git.kernel.org/stable/c/34e5667041050711a947e260fc9ebebe08bddee5 https://git.kernel.org/stable/c/459274c77b37ac63b78c928b4b4e748d1f9d05c8 https://git.kernel.org/stable/c/510603f504796c3535f67f55fb0b124a303b44c8 https://git.kernel.org/stable/c/893387c18612bb452336a5881da0d015a7e8f4a2 https://git.kernel.org/stable/c/9c23a90648e831d611152ac08dbcd1283d405e7f https://git.kernel.org/stable/c/ac60717f9a8d21c58617d0b34274babf24135835 https://git.kernel.org/stable/c/c08192b5d6730a914dee6175bc71092ee6a65f14 https://git.kernel.org/stable/c/d5490dfa35427a2967e00a4c7a1b95fdbc8ede34 https://git.kernel.org/stable/c/d7ddc93392e4a7ffcccc86edf6ef3e64c778db52 https://git.kernel.org/stable/c/f866eef8d1c65504d30923c3f14082ad294d0e6d

Frequently Asked Questions

What is CVE-2025-40300? +
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 ] It has a CVSS v3.1 base score of 5.5 (MEDIUM).
How severe is CVE-2025-40300? +
CVE-2025-40300 has a CVSS v3.1 score of 5.5 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.
What products are affected by CVE-2025-40300? +
CVE-2025-40300 affects products from debian, linux, specifically: debian_linux, linux_kernel. Check the affected products table above for specific version ranges.
How do I check if I'm vulnerable to CVE-2025-40300? +
You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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