linux-kernel - Re: [patch 05/24] x86/speculation: Disable STIBP when enhanced IBRS is in use

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Date:   Wed, 21 Nov 2018 21:33:53 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [patch 05/24] x86/speculation: Disable STIBP when enhanced IBRS
 is in use

On Wed, Nov 21, 2018 at 09:14:35PM +0100, Thomas Gleixner wrote:
> From: Tim Chen <tim.c.chen@...ux.intel.com>
> 
> If enhanced IBRS is active, STIBP is redundant for mitigating Spectre v2
> user space exploits from hyperthread sibling.
> 
> Disable STIBP when enhanced IBRS is used.
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> ---
>  arch/x86/kernel/cpu/bugs.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -321,6 +321,10 @@ static bool stibp_needed(void)
>  	if (spectre_v2_enabled == SPECTRE_V2_NONE)
>  		return false;
>  
> +	/* Enhanced IBRS makes using STIBP unnecessary. */
> +	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
> +		return false;
> +
>  	if (!boot_cpu_has(X86_FEATURE_STIBP))
>  		return false;
>  
> @@ -846,6 +850,9 @@ static ssize_t l1tf_show_state(char *buf
>  
>  static char *stibp_state(void)
>  {
> +	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
> +		return "";

If
	spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED

then SPEC_CTRL_STIBP should not be set in x86_spec_ctrl_base
(stibp_needed() prevents the setting in arch_smt_update()) so the above
check should not be needed.

I *think*.

-- 
Regards/Gruss,
    Boris.

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