Linux 6.9-rc4 To Bring New Fixes For x86 Speculation Mitigations

The Linux 6.9-rc4 weekly test release is due out later today and ahead of that this week's "x86/urgent" material has been sent in that includes several patches for various x86 speculation mitigation fixes.

For the Native BHI mitigation that landed last week for Intel processors there are a few fixes to that code with 6.9-rc4. The Branch History Injection (BHI) documentation has been cleared up to address some inaccuracies in the descriptions, there is a fix for BHI handling of RRSBA (Restricted RSB Alternate for newer Intel CPUs), and the spectre_bhi=auto / CONFIG_BHI_MITIGATION_AUTO path has been removed. The Spectre BHI "auto" mode doesn't make much sense since it only mitigates on newer Intel systems while leaving older Intel CPUs vulnerable. So it's better just relying on "spectre_bhi=on" instead of "auto".

More broadly in the x86 speculation mitigation world, Linux 6.9-rc4 has a fix to actually turn off the CPU security mitigations by default for kernel builds using the CONFIG_SPECULATION_MITIGATIONS Kconfig switch to force off the mitigations. A one-liner fix is needed since right now SPECULATION_MITIGATIONS=n on existing kernels still puts the kernel into the "auto" mitigation mode contrary to the Kconfig intention.

More details on this week's x86 urgent fixes via this pull ahead of Linux 6.9-rc4 being released in roughly twelve hours.