Re: [PATCH 1/2] x86/pat: fix x86_has_pat_wp()

[Borislav Petkov]

On Tue, May 03, 2022 at 03:22:06PM +0200, 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@xxxxxxxx>
> ---
>  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;

So this code always makes my head spin... especially after vacation but
lemme take a stab:

__pte2cachemode_tbl indices are of type enum page_cache_mode.

What you've done is index with

__cachemode2pte_tbl[_PAGE_CACHE_MODE_WP]

which gives uint16_t.

So, if at all, this should do __pte2cm_idx(_PAGE_CACHE_MODE_WP) to index
into it.

But I'm still unclear on the big picture. Looking at Jan's explanation,
there's something about PAT init being skipped due to MTRRs not being
emulated by Xen.... or something to that effect.

So if that's the case, the Xen guest code should init PAT in its own
way, so that the generic code works with this without doing hacks.

But I'm only guessing - this needs a *lot* more elaboration and
explanation why exactly this is needed.

Thx.

-- 
Regards/Gruss,
    Boris.

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