AMD Inception "SRSO" Mitigation Cleanup & Fixes Head To Linux 6.5-rc7

Written by Michael Larabel in AMD on 19 August 2023 at 06:28 AM EDT. 5 Comments
Earlier this month the AMD Inception vulnerability was disclosed and quickly mitigated within the mainline Linux kernel and back-ported to the stable kernels. In the rush to get the code merged and the mitigation being under embargo until the disclosure date, some bugs and clean-ups with the mitigation code were discovered. That revised code was now submitted today for merging ahead of the Linux 6.5-rc7 kernel release this weekend.

This AMD Inception / SRSO (Speculative Return Stack Overflow) mitigation code cleans up the new kernel code and fixes a few problems reported by users. The AMD Inception mitigation performance overhead isn't expected to change much at all from this cleansed code.

Inception vulnerability

AMD Linux engineer Borislav Petkov wrote in this morning's x86/urgent pull request:
"extraordinary embargoed times call for extraordinary measures. That's why this week's x86/urgent branch is larger than usual, containing all the known fallout fixes after the SRSO mitigation got merged.

I know, it is a bit late in the game but everyone who has reported a bug stemming from the SRSO pile, has tested that branch and has confirmed that it fixes their bug.

Also, I've run it on every possible hardware I have and it is looking good. It is running on this very machine while I'm typing, for 2 days now without an issue. Famous last words..."

Among the changes are ensuring the mitigation is disabled for unaffected configurations, correcting the mitigation status when SMT is disabled, and various code fixes courtesy of Intel's Peter Zijlstra. There is also a fix for the AMD Zen 1 divide by zero bug as part of this x86/urgent pull. The code should be merged to mainline in the coming hours and will also end up being back-ported to the Linux stable series for improving this security vulnerability mitigation.
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