linux-kernel - Re: [PATCH 4.4 29/63] x86/mm: Disable PCID on 32-bit kernels

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Date:   Tue, 09 Jan 2018 16:05:40 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        "Ghitulete, Razvan-alin" <rga@...zon.com>
Cc:     stable@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...nel.org>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 4.4 29/63] x86/mm: Disable PCID on 32-bit kernels

On Mon, 2018-01-01 at 15:24 +0100, Greg Kroah-Hartman wrote:
> 
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -19,6 +19,14 @@
>  
>  void __init check_bugs(void)
>  {
> +#ifdef CONFIG_X86_32
> +       /*
> +        * Regardless of whether PCID is enumerated, the SDM says
> +        * that it can't be enabled in 32-bit mode.
> +        */
> +       setup_clear_cpu_cap(X86_FEATURE_PCID);
> +#endif
> +
>         identify_boot_cpu();
>  #ifndef CONFIG_SMP
>         pr_info("CPU: ");
> 


Razvan points out that the #ifdef there is redundant; in older kernels,
bugs.c is only built on 32-bit anyway.

We're working on backporting the other CPU_BUG_* and sysfs
vulnerabilities bits to 4.9 (first), and will probably end up
cherry-picking 62a67e123e ("x86/cpu: Merge bugs.c and bugs_64.c").
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