linux-kernel - Re: [PATCH 01/22] x86/srso: Fix srso_show

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Date:   Mon, 21 Aug 2023 08:04:16 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Babu Moger <babu.moger@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, David.Kaplan@....com,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Nikolay Borisov <nik.borisov@...e.com>,
        gregkh@...uxfoundation.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 01/22] x86/srso: Fix srso_show_state() side effect

On Sun, Aug 20, 2023 at 06:18:58PM -0700, Josh Poimboeuf wrote:
> Reading the 'spec_rstack_overflow' sysfs file can trigger an unnecessary
> MSR write, and possibly even a (handled) exception if the microcode
> hasn't been updated.
> 
> Avoid all that by just checking X86_FEATURE_IBPB_BRTYPE instead, which
> gets set by srso_select_mitigation() if the updated microcode exists.
> 
> Fixes: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation")
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  arch/x86/kernel/cpu/bugs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index f081d26616ac..bdd3e296f72b 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -2717,7 +2717,7 @@ static ssize_t srso_show_state(char *buf)
>

Please put here a comment - something along the lines of:

"X86_FEATURE_IBPB_BRTYPE gets set as a result of the presence of the
needed microcode so checking that is equivalent."

so that it is clear why it is ok to check this feature bit.

>  	return sysfs_emit(buf, "%s%s\n",
>  			  srso_strings[srso_mitigation],
> -			  (cpu_has_ibpb_brtype_microcode() ? "" : ", no microcode"));
> +			  boot_cpu_has(X86_FEATURE_IBPB_BRTYPE) ? "" : ", no microcode");
>  }
>  
>  static ssize_t gds_show_state(char *buf)
> -- 

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette