linux-kernel - Re: [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page encryption bit

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Date:   Thu, 5 Sep 2019 07:15:07 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, pv-drivers@...are.com
Cc:     Thomas Hellstrom <thellstrom@...are.com>,
        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>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christian König <christian.koenig@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [RFC PATCH 1/2] x86: Don't let pgprot_modify() change the page
 encryption bit

Hi Thomas,

Thanks for the second batch of patches!  These look much improved on all
fronts.

On 9/5/19 3:35 AM, Thomas Hellström (VMware) wrote:
> -/* mprotect needs to preserve PAT bits when updating vm_page_prot */
> +/*
> + * mprotect needs to preserve PAT and encryption bits when updating
> + * vm_page_prot
> + */
>  #define pgprot_modify pgprot_modify
>  static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
>  {
> -	pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK;
> -	pgprotval_t addbits = pgprot_val(newprot);
> +	pgprotval_t preservebits = pgprot_val(oldprot) &
> +		(_PAGE_CHG_MASK | sme_me_mask);
> +	pgprotval_t addbits = pgprot_val(newprot) & ~sme_me_mask;
>  	return __pgprot(preservebits | addbits);
>  }

_PAGE_CHG_MASK is claiming similar functionality about preserving bits
when changing PTEs:

> /*
>  * Set of bits not changed in pte_modify.  The pte's
>  * protection key is treated like _PAGE_RW, for
>  * instance, and is *not* included in this mask since
>  * pte_modify() does modify it.
>  */
> #define _PAGE_CHG_MASK  (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |         \
>                          _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY | \
>                          _PAGE_SOFT_DIRTY | _PAGE_DEVMAP)

This makes me wonder if we should be including sme_me_mask in
_PAGE_CHG_MASK (logically).