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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →Spectre-v2 Branch History Injection (BHI) is a speculative-execution attack path in which manipulated branch history can influence an indirect-branch prediction in privileged code. If speculation reaches a suitable disclosure gadget, its cache effects may let an attacker infer sensitive kernel data. BHI is not a universal or ordinary read of kernel memory: whether a system is affected depends on its processor, microcode, kernel, and active mitigations.
What Branch History Injection means
BHI belongs to the Spectre variant 2 family. It involves the Branch History Buffer (BHB), which records recent branch behavior and can influence how a processor predicts an indirect branch—one whose destination is determined at runtime.
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Under the right conditions, attacker-influenced BHB state can steer a victim’s indirect-branch prediction toward a Branch Target Buffer (BTB) entry that is not associated with the victim branch’s source address. Linux’s Spectre documentation notes that this history can remain relevant even with enhanced IBRS (eIBRS): eIBRS can isolate predictor entries between privilege modes without isolating the BHB itself.
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- An attacker influences branch-history state.
- That history affects the target selected for a victim’s indirect branch.
- The CPU transiently follows the predicted path. For disclosure, that path must reach a useful gadget—code that accesses data of interest.
- Transient execution leaves cache effects. An attacker who can measure those effects may infer information, even though the speculative instructions did not commit architectural changes.
This is a speculative side channel, not a mechanism that simply grants arbitrary kernel-memory access. A useful gadget and measurable side effects are necessary to turn the prediction behavior into disclosure; the mechanism alone does not establish that every CPU or Linux installation is exploitable.
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Does eIBRS protect against BHI?
Not necessarily. eIBRS can restrict or isolate predictor entries across privilege modes, but it does not by itself eliminate the BHB influence described above. Linux documents software BHB clearing and, on supported processors, the hardware BHI_DIS_S control as mitigation approaches. Which protection is available and active depends on CPU support, vendor microcode, and the running kernel.
How Linux mitigates BHI
Linux documents two relevant full-mitigation approaches. The hardware approach uses BHI_DIS_S where supported; the software approach clears branch history with a software sequence. Availability and coverage can differ by CPU and context, including KVM. A CPU vendor microcode update may be required for full mitigation; Linux documentation says that if required microcode is unavailable, the kernel may report the system as vulnerable.
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| Approach | What it does | Availability and how to assess it |
|---|---|---|
| Hardware BHI_DIS_S | Uses a processor control to mitigate BHI. | Requires CPU support and, where necessary, vendor microcode. Check the running kernel’s reported status and CPU-vendor guidance. |
| Software BHB clearing | Uses a software sequence to clear branch-history state. | Used when appropriate for the CPU and kernel configuration. Check the reported status, including any KVM-specific state. |
The kernel generally chooses mitigations for the current CPU. Broader Spectre-v2 restrictions can carry performance overhead, but the cited Linux documentation does not give a BHI-specific performance figure.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsHow to check whether Linux reports BHI as mitigated
Read the running system’s vulnerability status rather than inferring protection from the CPU model or a boot parameter alone:
cat /sys/devices/system/cpu/vulnerabilities/spectre_v2Linux status wording can include BHI: Not affected, BHI: Retpoline, BHI: BHI_DIS_S, BHI: SW loop, KVM SW loop, BHI: Vulnerable, and BHI: Vulnerable, KVM: SW loop. Interpret the full output for the system and contexts in use; a KVM-specific indication matters if the machine runs virtual machines. The documented status descriptions are in the latest Linux Spectre documentation.
Status labels and mitigation behavior can evolve across kernel versions. The primary mechanism description is in upstream Linux’s next Spectre documentation; parameter behavior is also documented for Linux kernel 6.10. Use the documentation matching your running kernel and consult your processor vendor’s microcode guidance if the status reports vulnerability.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the spectre_bhi= parameter changes
The Linux kernel parameter spectre_bhi= controls deployment of hardware BHI control and the software BHB-clearing sequence. The documented configuration lists spectre_bhi=on as the default: the kernel enables hardware or software mitigation as needed. spectre_bhi=off disables the mitigation. The behavior is described in the Linux 6.10 kernel-parameters documentation.
Do not treat a boot setting as proof that a particular hardware control is supported or active. Confirm the status reported by the running kernel and follow CPU-vendor guidance; a required microcode update may affect whether full mitigation is available.
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