linux-kernel - Re: [patch V2 18/28] x86/speculation: Prepare for per task indirect branch speculation control

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Date:   Tue, 27 Nov 2018 21:52:33 +0000
From:   "Lendacky, Thomas" <Thomas.Lendacky@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.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>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        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>
Subject: Re: [patch V2 18/28] x86/speculation: Prepare for per task indirect
 branch speculation control

On 11/27/2018 02:42 PM, Thomas Gleixner wrote:
> On Tue, 27 Nov 2018, Thomas Gleixner wrote:
>> On Tue, 27 Nov 2018, Lendacky, Thomas wrote:
>>> On 11/25/2018 12:33 PM, Thomas Gleixner wrote:
>>>> --- a/arch/x86/kernel/process.c
>>>> +++ b/arch/x86/kernel/process.c
>>>> @@ -406,6 +406,11 @@ static __always_inline void spec_ctrl_up
>>>>  	if (static_cpu_has(X86_FEATURE_SSBD))
>>>>  		msr |= ssbd_tif_to_spec_ctrl(tifn);
>>>
>>> I did some quick testing and found my original logic was flawed. Since
>>> spec_ctrl_update_msr() can now be called for STIBP, an additional check
>>> is needed to set the SSBD MSR bit.
>>>
>>> Both X86_FEATURE_VIRT_SSBD and X86_FEATURE_LS_CFG_SSBD cause
>>> X86_FEATURE_SSBD to be set. Before this patch, spec_ctrl_update_msr() was
>>> only called if X86_FEATURE_SSBD was set and one of the other SSBD features
>>> wasn't set. But now, STIBP can cause spec_ctrl_update_msr() to get called
>>> and cause the SSBD MSR bit to be set when it shouldn't (could result in
>>> a GP fault).
>>
>> The below should fix that. We have the same logic in x86_virt_spec_ctrl()
> 
> Actually it's incomplete. Full version below.

Just one little nit on the comment below, otherwise works nicely.

Thanks,
Tom

> 
> Thanks,
> 
> 	tglx
> 
> 8<-----------------
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -403,10 +403,11 @@ static __always_inline void spec_ctrl_up
>  	u64 msr = x86_spec_ctrl_base;
>  
>  	/*
> -	 * If X86_FEATURE_SSBD is not set, the SSBD bit is not to be
> -	 * touched.
> +	 * If SSBD is not controlled in MSR_SPEC_CTRL, the SSBD bit has not

s/has not/is not/

> +	 * to be touched.
>  	 */
> -	if (static_cpu_has(X86_FEATURE_SSBD))
> +	if (static_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD) ||
> +	    static_cpu_has(X86_FEATURE_AMD_SSBD))
>  		msr |= ssbd_tif_to_spec_ctrl(tifn);
>  
>  	/* Only evaluate if conditional STIBP is enabled */
> @@ -440,7 +441,8 @@ static __always_inline void __speculatio
>  			amd_set_ssb_virt_state(tifn);
>  		else if (static_cpu_has(X86_FEATURE_LS_CFG_SSBD))
>  			amd_set_core_ssb_state(tifn);
> -		else if (static_cpu_has(X86_FEATURE_SSBD))
> +		else if (static_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD) ||
> +			 static_cpu_has(X86_FEATURE_AMD_SSBD))
>  			updmsr  = true;
>  	}
>  
>