Re: General protection fault in `switch_mm_irqs_off()`

Borislav Petkov
2019-01-09T21:11:21Z

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.

Good mailing practices for 400: avoid top-posting and trim the reply.