linux-kernel - Re: [PATCH v1 1/2] x86/retpoline: Add a function to clear the RETPOLINE

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Date:   Tue, 9 Jan 2018 17:38:19 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Rik van Riel <riel@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v1 1/2] x86/retpoline: Add a function to clear the
 RETPOLINE_AMD feature

On 1/9/2018 5:09 PM, Thomas Gleixner wrote:
> On Tue, 9 Jan 2018, Tom Lendacky wrote:
>> On 1/9/2018 4:46 PM, Thomas Gleixner wrote:
>>> Urgh. That's an awful hack. why not do the obvious?
>>
>> My first attempt was very similar to your change below, but testing
>> showed that spectre_v2_check_boottime_disable() is called before the
>> X86_FEATURE_LFENCE_RDTSC can be set.  I can look at moving where the
>> X86_FEATURE_LFENCE_RDTSC is set, maybe into early_init_amd() or such
>> if you think that would be best.
> 
> Wait, we can move the selection _AFTER_ identify_boot_cpu().

Much cleaner.  Since it's just a single patch now, do you want me to
re-submit this after I test it or will you just pick this up as is?

I did notice that the patch does change the behavior associated with
the command line options, though.  Not sure if that was intentional.

Thanks,
Tom

> 
> Thanks,
> 
> 	tglx
> 
> 8<----------------
> 
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -152,7 +152,5 @@
>  # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
>  #endif
>  
> -void spectre_v2_check_boottime_disable(void);
> -
>  #endif /* __ASSEMBLY__ */
>  #endif /* __NOSPEC_BRANCH_H__ */
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -24,6 +24,8 @@
>  #include <asm/pgtable.h>
>  #include <asm/set_memory.h>
>  
> +static void __init spectre_v2_check_boottime_disable(void);
> +
>  void __init check_bugs(void)
>  {
>  	identify_boot_cpu();
> @@ -33,6 +35,9 @@ void __init check_bugs(void)
>  		print_cpu_info(&boot_cpu_data);
>  	}
>  
> +	/* Select the proper spectre mitigation before patching alternatives */
> +	spectre_v2_check_boottime_disable();
> +
>  #ifdef CONFIG_X86_32
>  	/*
>  	 * Check whether we are able to run this kernel safely on SMP.
> @@ -106,7 +111,7 @@ static inline bool match_option(const ch
>  	return len == arglen && !strncmp(arg, opt, len);
>  }
>  
> -void __init spectre_v2_check_boottime_disable(void)
> +static void __init spectre_v2_check_boottime_disable(void)
>  {
>  	char arg[20];
>  	int ret;
> @@ -148,14 +153,21 @@ void __init spectre_v2_check_boottime_di
>  retpoline:
>  	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
>  	retpoline_amd:
> +		if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD ||
> +		    !boot_cpu_has(X86_FEATURE_LFENCE_RDTSC)) {
> +			pr_info("AMD retpoline not supported, fall back to generic\n");
> +			goto retpoline_generic;
> +		}
> +
>  		spectre_v2_enabled = retp_compiler() ?
>  			SPECTRE_V2_RETPOLINE_AMD : SPECTRE_V2_RETPOLINE_MINIMAL_AMD;
>  		setup_force_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
> -	} else {
> -	retpoline_generic:
> -		spectre_v2_enabled = retp_compiler() ?
> -			SPECTRE_V2_RETPOLINE_GENERIC : SPECTRE_V2_RETPOLINE_MINIMAL;
> +		setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
> +		return;
>  	}
> +retpoline_generic:
> +	spectre_v2_enabled = retp_compiler() ?
> +		SPECTRE_V2_RETPOLINE_GENERIC : SPECTRE_V2_RETPOLINE_MINIMAL;
>  	setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
>  	return;
>  #else
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1322,8 +1322,6 @@ void __init setup_arch(char **cmdline_p)
>  
>  	register_refined_jiffies(CLOCK_TICK_RATE);
>  
> -	spectre_v2_check_boottime_disable();
> -
>  #ifdef CONFIG_EFI
>  	if (efi_enabled(EFI_BOOT))
>  		efi_apply_memmap_quirks();
>