linux-kernel - Re: [PATCH v4 11/39] x86/mm: Update pte_modify for _PAGE

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Message-ID: <Y6raFBB+oVx+2WXl@zn.tnic>
Date:   Tue, 27 Dec 2022 12:42:12 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v4 11/39] x86/mm: Update pte_modify for _PAGE_COW

On Fri, Dec 02, 2022 at 04:35:38PM -0800, Rick Edgecombe wrote:
>  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>  {
> +	pteval_t _page_chg_mask_no_dirty = _PAGE_CHG_MASK & ~_PAGE_DIRTY;
>  	pteval_t val = pte_val(pte), oldval = val;
> +	pte_t pte_result;
>  
>  	/*
>  	 * Chop off the NX bit (if present), and add the NX portion of
>  	 * the newprot (if present):
>  	 */
> -	val &= _PAGE_CHG_MASK;
> -	val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
> +	val &= _page_chg_mask_no_dirty;
> +	val |= check_pgprot(newprot) & ~_page_chg_mask_no_dirty;
>  	val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);
> -	return __pte(val);
> +
> +	pte_result = __pte(val);
> +
> +	/*
> +	 * Dirty bit is not preserved above so it can be done

Just for my own understanding: are you saying here that
flip_protnone_guard() might end up setting _PAGE_DIRTY in val...

> +	 * in a special way for the shadow stack case, where it
> +	 * needs to set _PAGE_COW. pte_mkcow() will do this in
> +	 * the case of shadow stack.
> +	 */
> +	if (pte_dirty(pte_result))
> +		pte_result = pte_mkcow(pte_result);

... and in that case we need to turn it into a _PAGE_COW setting?

Thx.

-- 
Regards/Gruss,
    Boris.

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