linux-kernel - Re: General protection fault in `switch_mm_irqs

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Date:   Wed, 9 Jan 2019 22:11:04 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Paul Menzel <pmenzel@...gen.mpg.de>
Cc:     Thomas Lendacky <Thomas.Lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Kosina <jikos@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: General protection fault in `switch_mm_irqs_off()`

On Wed, Jan 09, 2019 at 05:34:11PM +0100, Paul Menzel wrote:
> Is there a way to trace the value of `boot_cpu_data` from
> `arch/x86/include/asm/cpufeature.h` with some Linux Kernel magic?
> 
>     #define boot_cpu_has(bit)       cpu_has(&boot_cpu_data, bit)
> 
> Or is rebuilding with print statements the only solution?

Yes. Just apply this and catch output. It is a wild guess anyway as
this whole deal looks really strange but at least it should not #GP the
machine.

---
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index dad12b767ba0..ec4688779900 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -284,6 +284,9 @@ static inline void indirect_branch_prediction_barrier(void)
 {
 	u64 val = PRED_CMD_IBPB;
 
+	if (WARN_ON(boot_cpu_has(X86_FEATURE_USE_IBPB)))
+		return;
+
 	alternative_msr_write(MSR_IA32_PRED_CMD, val, X86_FEATURE_USE_IBPB);
 }
 
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 1de0f4170178..4ed4cc99a2c0 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -371,6 +371,8 @@ spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd)
 	if (boot_cpu_has(X86_FEATURE_IBPB)) {
 		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
 
+		pr_err("%s: set X86_FEATURE_USE_IBPB\n", __func__);
+
 		switch (cmd) {
 		case SPECTRE_V2_USER_CMD_FORCE:
 		case SPECTRE_V2_USER_CMD_PRCTL_IBPB:

-- 
Regards/Gruss,
    Boris.

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