linux-kernel - Re: [PATCH v2 6/8] x86/microcode: Recheck IBRS features on microcode reload

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Date:   Tue, 9 Jan 2018 01:34:54 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/8] x86/microcode: Recheck IBRS features on microcode
 reload

On Fri, Jan 05, 2018 at 06:12:21PM -0800, Tim Chen wrote:
> On new microcode write, check whether IBRS
> is present by rescanning IBRS feature.
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>  arch/x86/include/asm/spec_ctrl.h     |  1 +
>  arch/x86/kernel/cpu/microcode/core.c |  4 ++++
>  arch/x86/kernel/cpu/spec_ctrl.c      | 18 ++++++++++--------
>  3 files changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/include/asm/spec_ctrl.h b/arch/x86/include/asm/spec_ctrl.h
> index 62c5dc8..be08ae7 100644
> --- a/arch/x86/include/asm/spec_ctrl.h
> +++ b/arch/x86/include/asm/spec_ctrl.h
> @@ -8,6 +8,7 @@
>  #include <asm/microcode.h>
>  
>  void scan_spec_ctrl_feature(struct cpuinfo_x86 *c);
> +void rescan_spec_ctrl_feature(struct cpuinfo_x86 *c);
>  bool ibrs_inuse(void);
>  
>  extern unsigned int dynamic_ibrs;
> diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
> index c4fa4a8..848c3e9 100644
> --- a/arch/x86/kernel/cpu/microcode/core.c
> +++ b/arch/x86/kernel/cpu/microcode/core.c
> @@ -40,6 +40,7 @@
>  #include <asm/processor.h>
>  #include <asm/cmdline.h>
>  #include <asm/setup.h>
> +#include <asm/spec_ctrl.h>
>  
>  #define DRIVER_VERSION	"2.2"
>  
> @@ -444,6 +445,9 @@ static ssize_t microcode_write(struct file *file, const char __user *buf,
>  	if (ret > 0)
>  		perf_check_microcode();
>  
> +	/* check spec_ctrl capabilities */
> +	rescan_spec_ctrl_feature(&boot_cpu_data);

Needs to be under the if like perf_check_microcode().

> +
>  	mutex_unlock(&microcode_mutex);
>  	put_online_cpus();
>  
> diff --git a/arch/x86/kernel/cpu/spec_ctrl.c b/arch/x86/kernel/cpu/spec_ctrl.c
> index 1641bec..28107a2 100644
> --- a/arch/x86/kernel/cpu/spec_ctrl.c
> +++ b/arch/x86/kernel/cpu/spec_ctrl.c
> @@ -24,13 +24,18 @@ static bool ibrs_admin_disabled;
>  /* mutex to serialize IBRS control changes */
>  DEFINE_MUTEX(spec_ctrl_mutex);
>  
> +static inline void set_ibrs_feature(void)
> +{
> +	if (!ibrs_admin_disabled) {
> +		dynamic_ibrs = 1;
> +		ibrs_enabled = IBRS_ENABLED;
> +	}
> +}
> +
>  void scan_spec_ctrl_feature(struct cpuinfo_x86 *c)
>  {
>  	if ((!c->cpu_index) && (boot_cpu_has(X86_FEATURE_SPEC_CTRL))) {
> -		if (!ibrs_admin_disabled) {
> -			dynamic_ibrs = 1;
> -			ibrs_enabled = IBRS_ENABLED;
> -		}
> +		set_ibrs_feature();
>  	}
>  }
>  EXPORT_SYMBOL_GPL(scan_spec_ctrl_feature);
> @@ -43,10 +48,7 @@ void rescan_spec_ctrl_feature(struct cpuinfo_x86 *c)
>  {
>  	mutex_lock(&spec_ctrl_mutex);
>  	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) {
> -		if (!ibrs_admin_disabled) {
> -			dynamic_ibrs = 1;
> -			ibrs_enabled = IBRS_ENABLED;
> -		}
> +		set_ibrs_feature();

As already stated, this does not work. You want to check the CPUID leaf
after the microcode reload and then set X86_FEATURE_SPEC_CTRL on each
CPU.

-- 
Regards/Gruss,
    Boris.

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