Linux Landing Change To Allow STIBP When Using Legacy IBRS

Written by Michael Larabel in Linux Security on 5 March 2023 at 06:31 AM EST. 2 Comments
Ahead of the Linux 6.3-rc1 release later today, a set of "x86/urgent" patches were sent out Sunday morning that include the change to allow Single Threaded Indirect Branch Predictors (STIBP) to be used in the presence of legacy Indirect Branch Restricted Speculation (IBRS) for security reasons.

Since a change last year, the Linux kernel hasn't enabled STIBP when using IBRS. However, the performance-hurting legacy IBRS is cleared on return to user-space for performance reasons but in turn that leaves user-space vulnerable to cross-thread attacks. So STIBP can now be ena bled while having legacy IBRS active in order to fend off those user-space attacks.

Core i9 9900K
Legacy IBRS is used in just older Intel CPUs prior to the introduction of eIBRS.

I wrote about this issue in more detail last month in Linux Inadvertently Has Been Leaving IBRS-Mitigated Systems Without STIBP.

Now that it's been sent in via x86/urgent it should land today ahead of Linux 6.3-rc1. This change should also in turn be back-ported to supported stable kernel series as well in the days ahead.
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