To mitigate Spectre-v2 Branch History Injection (BHI), install the latest supported kernel update for your Linux distribution, apply any applicable CPU microcode or firmware updates through supported channels, reboot, and check the kernel’s BHI status. There is no universal package command or kernel version: the right update depends on your distribution and release, CPU, kernel flavor, and whether the system is a host, guest, or hypervisor. To answer “How do I update Linux to mitigate Spectre-v2 BHI attacks?”, follow your distribution’s current update guidance, then verify the result rather than assuming a kernel update alone provides full protection.
What BHI is—and why updating matters
Branch History Injection is a Spectre variant 2 attack path. It poisons the processor’s Branch History Buffer (BHB) to influence indirect branch prediction toward a Branch Target Buffer (BTB) entry that need not match the indirect branch’s source. Because branch history can be shared across privilege levels, the attack can matter even on systems with Enhanced IBRS.
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The Linux kernel’s Spectre documentation recommends BHI_DIS_S or a BHB clearing sequence for full protection against BHB attacks. The kernel generally chooses mitigations appropriate to the CPU, but full mitigation may depend on CPU-vendor microcode that is not present on every system. See the Linux kernel Spectre documentation.
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Update and verify Linux BHI protection
- Identify the system. Note your Linux distribution and release, CPU architecture and model, kernel flavor, and whether the machine is a physical host, a virtual machine guest, or a hypervisor. Update instructions and exposure can differ across these cases.
- Install supported updates. Use the distribution’s normal security and kernel update channel for your release. Also install applicable CPU microcode or firmware updates using the distribution’s or system vendor’s supported mechanism. Do not rely on an old package version from a past security notice as a current fix.
- Reboot into the updated kernel. A package can be installed without the system yet running it. After reboot, confirm that the running kernel is the updated one using your distribution’s usual system tools.
- Check the kernel’s report. Run
cat /sys/devices/system/cpu/vulnerabilities/spectre_v2. Read the BHI portion of the output; the kernel documents states includingNot affected,Retpoline,BHI_DIS_S, software-loop protection, and vulnerable states. The status interface and its interpretation are described in the kernel documentation. - Respond to a vulnerable result. Check for further supported kernel, microcode, firmware, or hypervisor updates relevant to that machine. If the system is a VM, the host or hypervisor may also need attention; updating only the guest does not establish the host’s mitigation state.
How to interpret the BHI status
A reported BHI state such as BHI_DIS_S or a software loop indicates that the kernel reports a mitigation path; Not affected indicates the kernel considers the CPU not affected. A Vulnerable result means the kernel reports exposure, which can involve a component such as KVM. The precise strings and detail vary with CPU and kernel support, so interpret the whole output rather than matching only one word.
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The status file is a useful check of the kernel’s Spectre-v2 mitigation state, not proof that every speculative-execution attack is impossible. A 2024 USENIX Security paper on native BHI described kernel-memory disclosure attacks and reported bypassing deployed mitigations in its research; that work is context for the limits of any single status string, not a reason to disregard upstream Linux mitigation guidance. See the USENIX Security 2024 paper.
Why there is no single update command or version
Package managers, supported kernel branches, kernel flavors, and firmware delivery vary by distribution and release. The kernel’s mitigation behavior also depends on CPU capabilities and available microcode. Use the current instructions for your exact distribution release instead of copying a command or package version written for another system.
Ubuntu’s BHI guidance recommends moving to the latest kernel, but its listed package versions refer to March 2022 and are historical, not a current version list. Consult the Ubuntu BHI guidance for context, then follow your release’s current update channel.
Should you change Spectre kernel boot options?
Usually, no. Linux provides controls such as spectre_v2= and spectre_bhi=, but the kernel documentation says it generally selects reasonable default mitigations for the CPU. Do not disable or override mitigations to chase performance without authoritative, platform-specific guidance; an override can change the protection in effect. The available controls and their behavior are documented by the Linux kernel.
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