linux-kernel - Re: [PATCH 1/2] x86/pat: fix x86_has_pat

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Date:   Tue, 14 Jun 2022 17:09:55 +0200
From:   Juergen Gross <jgross@...e.com>
To:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     jbeulich@...e.com, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/2] x86/pat: fix x86_has_pat_wp()

On 03.05.22 15:22, Juergen Gross wrote:
> x86_has_pat_wp() is using a wrong test, as it relies on the normal
> PAT configuration used by the kernel. In case the PAT MSR has been
> setup by another entity (e.g. BIOS or Xen hypervisor) it might return
> false even if the PAT configuration is allowing WP mappings.
> 
> Fixes: 1f6f655e01ad ("x86/mm: Add a x86_has_pat_wp() helper")
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>   arch/x86/mm/init.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index d8cfce221275..71e182ebced3 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -80,7 +80,8 @@ static uint8_t __pte2cachemode_tbl[8] = {
>   /* Check that the write-protect PAT entry is set for write-protect */
>   bool x86_has_pat_wp(void)
>   {
> -	return __pte2cachemode_tbl[_PAGE_CACHE_MODE_WP] == _PAGE_CACHE_MODE_WP;
> +	return __pte2cachemode_tbl[__cachemode2pte_tbl[_PAGE_CACHE_MODE_WP]] ==
> +	       _PAGE_CACHE_MODE_WP;
>   }
>   
>   enum page_cache_mode pgprot2cachemode(pgprot_t pgprot)

x86 maintainers, please consider taking this patch, as it is fixing
a real bug. Patch 2 of this series can be dropped IMO.


Juergen

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