Re: [PATCH v3 04/19] x86/intel: Initialize IA32_FEATURE_CONTROL MSR at boot

Kai Huang
2019-11-19T04:42:07Z

On Mon, 2019-11-18 at 19:12 -0800, Sean Christopherson wrote:
> Opportunistically initialize IA32_FEATURE_CONTROL MSR to enable VMX when
> the MSR is left unlocked by BIOS. Configuring IA32_FEATURE_CONTROL at
> boot time paves the way for similar enabling of other features, e.g.
> Software Guard Extensions (SGX).
>
> Temporarily leave equivalent KVM code in place in order to avoid
> introducing a regression on Centaur and Zhaoxin CPUs, e.g. removing
> KVM's code would leave the MSR unlocked on those CPUs and would break
> existing functionality if people are loading kvm_intel on Centaur and/or
> Zhaoxin. Defer enablement of the boot-time configuration on Centaur and
> Zhaoxin to future patches to aid bisection.
>
> Note, Local Machine Check Exceptions (LMCE) are also supported by the
> kernel and enabled via IA32_FEATURE_CONTROL, but the kernel currently
> uses LMCE if and and only if the feature is explicitly enabled by BIOS.
> Keep the current behavior to avoid introducing bugs, future patches can
> opt in to opportunistic enabling if it's deemed desirable to do so.
>
> Always lock IA32_FEATURE_CONTROL if it exists, even if the CPU doesn't
> support VMX, so that other existing and future kernel code that queries
> IA32_FEATURE_CONTROL can assume it's locked.
>
> Start from a clean slate when constructing the value to write to
> IA32_FEATURE_CONTROL, i.e. ignore whatever value BIOS left in the MSR so
> as not to enable random features or fault on the WRMSR.
>
> Suggested-by: Borislav Petkov <bp@suse.de>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> arch/x86/Kconfig.cpu | 4 +++
> arch/x86/kernel/cpu/Makefile | 1 +
> arch/x86/kernel/cpu/cpu.h | 4 +++
> arch/x86/kernel/cpu/feature_control.c | 35 +++++++++++++++++++++++++++
> arch/x86/kernel/cpu/intel.c | 2 ++
> 5 files changed, 46 insertions(+)
> create mode 100644 arch/x86/kernel/cpu/feature_control.c
>
> diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
> index af9c967782f6..aafc14a0abf7 100644
> --- a/arch/x86/Kconfig.cpu
> +++ b/arch/x86/Kconfig.cpu
> @@ -387,6 +387,10 @@ config X86_DEBUGCTLMSR
> def_bool y
> depends on !(MK6 || MWINCHIPC6 || MWINCHIP3D || MCYRIXIII || M586MMX ||
> M586TSC || M586 || M486SX || M486) && !UML
>
> +config X86_FEATURE_CONTROL_MSR
> + def_bool y
> + depends on CPU_SUP_INTEL
> +
> menuconfig PROCESSOR_SELECT
> bool "Supported processor vendors" if EXPERT
> ---help---
> diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
> index 890f60083eca..84e35e762f76 100644
> --- a/arch/x86/kernel/cpu/Makefile
> +++ b/arch/x86/kernel/cpu/Makefile
> @@ -29,6 +29,7 @@ obj-y += umwait.o
> obj-$(CONFIG_PROC_FS) += proc.o
> obj-$(CONFIG_X86_FEATURE_NAMES) += capflags.o powerflags.o
>
> +obj-$(CONFIG_X86_FEATURE_CONTROL_MSR) += feature_control.o
> ifdef CONFIG_CPU_SUP_INTEL
> obj-y += intel.o intel_pconfig.o tsx.o
> obj-$(CONFIG_PM) += intel_epb.o
> diff --git a/arch/x86/kernel/cpu/cpu.h b/arch/x86/kernel/cpu/cpu.h
> index 38ab6e115eac..a58e80866a3f 100644
> --- a/arch/x86/kernel/cpu/cpu.h
> +++ b/arch/x86/kernel/cpu/cpu.h
> @@ -80,4 +80,8 @@ extern void x86_spec_ctrl_setup_ap(void);
>
> extern u64 x86_read_arch_cap_msr(void);
>
> +#ifdef CONFIG_X86_FEATURE_CONTROL_MSR
> +void init_feature_control_msr(struct cpuinfo_x86 *c);
> +#endif
> +
> #endif /* ARCH_X86_CPU_H */
> diff --git a/arch/x86/kernel/cpu/feature_control.c
> b/arch/x86/kernel/cpu/feature_control.c
> new file mode 100644
> index 000000000000..33c9444dda52
> --- /dev/null
> +++ b/arch/x86/kernel/cpu/feature_control.c
> @@ -0,0 +1,35 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/tboot.h>
> +
> +#include <asm/cpufeature.h>
> +#include <asm/msr-index.h>
> +#include <asm/processor.h>
> +
> +void init_feature_control_msr(struct cpuinfo_x86 *c)
> +{
> + u64 msr;
> +
> + if (rdmsrl_safe(MSR_IA32_FEATURE_CONTROL, &msr))
> + return;
> +
> + if (msr & FEAT_CTL_LOCKED)
> + return;
> +
> + /*
> + * Ignore whatever value BIOS left in the MSR to avoid enabling random
> + * features or faulting on the WRMSR.
> + */
> + msr = FEAT_CTL_LOCKED;
> +
> + /*
> + * Enable VMX if and only if the kernel may do VMXON at some point,
> + * i.e. KVM is enabled, to avoid unnecessarily adding an attack vector
> + * for the kernel, e.g. using VMX to hide malicious code.
> + */
> + if (cpu_has(c, X86_FEATURE_VMX) && IS_ENABLED(CONFIG_KVM)) {
> + msr |= FEAT_CTL_VMX_ENABLED_OUTSIDE_SMX;
> + if (tboot_enabled())
> + msr |= FEAT_CTL_VMX_ENABLED_INSIDE_SMX;
> + }

Why not also take this chance to enable SGX? Or it will come with SGX patch
series?

> + wrmsrl(MSR_IA32_FEATURE_CONTROL, msr);
> +}
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 4a900804a023..b7c6ed0b40b6 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -755,6 +755,8 @@ static void init_intel(struct cpuinfo_x86 *c)
> /* Work around errata */
> srat_detect_node(c);
>
> + init_feature_control_msr(c);

Will this compile if you disable CONFIG_X86_FEATURE_CONTROL_MSR?

Provide an empty one in cpu.h if the config is not enabled?

Thanks,
-Kai
> +
> if (cpu_has(c, X86_FEATURE_VMX))
> detect_vmx_virtcap(c);
>