linux-kernel - Re: [PATCH v2] x86/pat: fix x86_has_pat

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Date:   Mon, 20 Jun 2022 15:22:08 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>,
        Tom Lendacky <thomas.lendacky@....com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] x86/pat: fix x86_has_pat_wp()

+ Tom.

On Mon, Jun 20, 2022 at 01:34:41PM +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.

... because Xen doesn't allow writing the PAT MSR. Please explain
exactly what happens because we will forget.

> The correct way to test for WP support is:
> 
> 1. Get the PTE protection bits needed to select WP mode by reading
>    __cachemode2pte_tbl[_PAGE_CACHE_MODE_WP] (depending on the PAT MSR
>    setting this might return protection bits for a stronger mode, e.g.
>    UC-)
> 2. Translate those bits back into the real cache mode selected by those
>    PTE bits by reading __pte2cachemode_tbl[__pte2cm_idx(prot)]
> 3. Test for the cache mode to be _PAGE_CACHE_MODE_WP

Yes, this is a good explanation albeit a bit too verbose. You can stick
a shorter version of it as a comment over the function so that we don't
have to swap it all back in next time.

> Fixes: 1f6f655e01ad ("x86/mm: Add a x86_has_pat_wp() helper")

If anything, this should be:

f88a68facd9a ("x86/mm: Extend early_memremap() support with additional attrs")

Also, I'm thinking CC:stable here.

> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
> V2:
> - fix indexing into __pte2cachemode_tbl[]

Yes, in any case, I see it now. The key aspect being in the comment
above it:

 *   Index into __pte2cachemode_tbl[] are the caching attribute bits of the pte
 *   (_PAGE_PWT, _PAGE_PCD, _PAGE_PAT) at index bit positions 0, 1, 2.

which is how one should index into that array.

Thx.

-- 
Regards/Gruss,
    Boris.

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