Messages in this thread Patch in this message | | | Date | Mon, 15 Jun 2020 12:47:13 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/speculation/srbds: do not try to turn mitigation off when not supported |
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On Mon, Jun 15, 2020 at 07:27:38AM -0300, Thadeu Lima de Souza Cascardo wrote: > It was booted without the microcode update. There was microcode available, but > systems may be booted without it, thus causing warnings due to the MSR > read/write.
Right.
> That's exactly the fix in the patch I sent, right? Do you want me to resend > with a comment, then?
Your patch replaced
- if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED)
Thinking about this more, I think the proper thing to do is this:
--- diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 0b71970d2d3d..ce2931563f8f 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -432,14 +432,14 @@ void update_srbds_msr(void) if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) return; - if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED) + if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED || + srbds_mitigation == SRBDS_MITIGATION_TSX_OFF) return; rdmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl); switch (srbds_mitigation) { case SRBDS_MITIGATION_OFF: - case SRBDS_MITIGATION_TSX_OFF: mcu_ctrl |= RNGDS_MITG_DIS; break; case SRBDS_MITIGATION_FULL: --- because looking at:
7e5b3c267d25 ("x86/speculation: Add Special Register Buffer Data Sampling (SRBDS) mitigation")
it says:
While it is present on all affected CPU models, the microcode mitigation is not needed on models that enumerate ARCH_CAPABILITIES[MDS_NO] in the cases where TSX is not supported or has been disabled with TSX_CTRL.
which could be also understood as "there won't be microcode for those CPUs and thus MSR_IA32_MCU_OPT_CTRL won't be there."
Because if that is the case, then SRBDS_MITIGATION_TSX_OFF means the MSR is not there and therefore we should not touch it.
And you've actually shown that without the microcode loaded, you can have a system which is MDS_NO and which hasn't generated MSR_IA32_MCU_OPT_CTRL (because the microcode is not loaded) and thus can't touch said MSR.
Mark?
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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