Hello,
I have noticed on my zen3 and zen4 machines (Ryzen 9 5950x & Ryzen 9 7950x) that the kernel boot log regarding SRSO is suddenly incorrect with the 6.6.2 kernel.
I have observed this on a completely mainline/stable kernel that I compile regularly for my machines. With the 6.6.1 and 6.7-rc2 kernels, I see correct boot messages like this:
[ 0.161327] Spectre V1 : Mitigation: usercopy/swapgs barriers and __user pointer sanitization [ 0.161329] Spectre V2 : Mitigation: Retpolines [ 0.161330] Spectre V2 : Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch [ 0.161331] Spectre V2 : Spectre v2 / SpectreRSB : Filling RSB on VMEXIT [ 0.161332] Spectre V2 : Enabling Restricted Speculation for firmware calls [ 0.161333] Spectre V2 : mitigation: Enabling conditional Indirect Branch Prediction Barrier [ 0.161335] Spectre V2 : User space: Mitigation: STIBP always-on protection [ 0.161336] Speculative Store Bypass: Mitigation: Speculative Store Bypass disabled via prctl [ 0.161338] Speculative Return Stack Overflow: Mitigation: safe RET
However, with the 6.6.2 kernel, I see this:
[ 0.164266] Spectre V1 : Mitigation: usercopy/swapgs barriers and __user pointer sanitization [ 0.164268] Spectre V2 : Mitigation: Retpolines [ 0.164269] Spectre V2 : Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch [ 0.164270] Spectre V2 : Spectre v2 / SpectreRSB : Filling RSB on VMEXIT [ 0.164272] Spectre V2 : Enabling Restricted Speculation for firmware calls [ 0.164273] Spectre V2 : mitigation: Enabling conditional Indirect Branch Prediction Barrier [ 0.164275] Spectre V2 : User space: Mitigation: STIBP always-on protection [ 0.164276] Speculative Store Bypass: Mitigation: Speculative Store Bypass disabled via prctl [ 0.164278] Speculative Return Stack Overflow: IBPB-extending microcode not applied! [ 0.164279] Speculative Return Stack Overflow: WARNING: See https://kernel.org/doc/html/latest/admin-guide/hw-vuln/srso.html for mitigation options. [ 0.164280] Speculative Return Stack Overflow: Mitigation: Safe RET
Notice that in both cases the final SRSO message indicates (correctly) that my system is protected with Safe RET (because my BIOS has the updated microcode for SRSO). However, in the 6.6.2 kernel I also get the microcode warning.
I tracked this down to, what I believe is, the following commit from the mainline kernel not being included in the 6.6.2 patch set: 351236947a45a512c517153bbe109fe868d05e6d x86/srso: Move retbleed IBPB check into existing 'has_microcode' code block
When I cherry-pick this commit to the 6.6.2 release, the log messages are correct. Note that this patch does not obviously indicate there is a functional change introduced by applying it. However (at least in the case of Zen3 and Zen4) when the updated microcode has been applied, the flow is different (specifically the situation that falls into the else statement that produces the pr-warn calls to indicate that the microcode fix needs applied). I believe this might be because RETBLEED does not apply to Zen3 and Zen4. Unfortunately I don't have a Zen1 or Zen2 system readily available on which to test this theory.
While this should only be a cosmetic change, I believe that there is value in having the correct messages in the kernel logs for SRSO in this LTS release.
Please feel free to correct my analysis above if it is incorrect.
Warm Regards,
Caleb Jorden