linux-kernel - Re: [RFC PATCH v9 10/27] x86/mm: Update pte_modify, pmd_modify, and _PAGE_CHG_MASK for _PAGE_DIRTY

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Date:   Wed, 26 Feb 2020 14:02:06 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>, 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>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>, x86-patch-review@...el.com
Subject: Re: [RFC PATCH v9 10/27] x86/mm: Update pte_modify, pmd_modify, and
 _PAGE_CHG_MASK for _PAGE_DIRTY_SW

The subject really needs work.  Could you think of a way to summarize
the changes here in english as opposed to just listing the symbols you
modified?

I think we could probably just auto-generate subjects for patches if the
existing one were sufficient.

On 2/5/20 10:19 AM, Yu-cheng Yu wrote:
> After the introduction of _PAGE_DIRTY_SW, pte_modify and pmd_modify need to
> set the Dirty bit accordingly: if Shadow Stack is enabled and _PAGE_RW is
> cleared, use _PAGE_DIRTY_SW; otherwise _PAGE_DIRTY_HW.

You've basically gone and written the code's if() statement in english
here.  That doesn't really help me understand the patch.

> Since the Dirty bit is modify by pte_modify(), remove _PAGE_DIRTY_HW from
> PAGE_CHG_MASK.

			 ^ modified

This is a great example of a changelog that adds very little value.
It's following the comments and doing what they say, but it's pretty
obvious that the analysis stopped there.

What *kinds* of bits are in _PAGE_CHG_MASK or not?  What changed about
_PAGE_DIRTY_HW.  By this definition, shouldn't _PAGE_DIRTY_SW have
technically been in this mask before this patch?

> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 62aeb118bc36..2733e7ec16b3 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -702,6 +702,14 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>  	val &= _PAGE_CHG_MASK;
>  	val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
>  	val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);
> +
> +	if (pte_dirty(pte)) {
> +		if (static_cpu_has(X86_FEATURE_SHSTK) && !(val & _PAGE_RW))
> +			val |= _PAGE_DIRTY_SW;
> +		else
> +			val |= _PAGE_DIRTY_HW;
> +	}
> +
>  	return __pte(val);
>  }

OK, so this is a path we use for changing bunches of PTEs to 'newprot'.
 It doesn't use the pte_*() helpers that the previous patch fixed up, so
we need a new site.

Right?

Maybe that would make good changelog text.

Also, couldn't we just have a pte_fixup() function or something that did
this logic and could be shared?

> @@ -712,6 +720,14 @@ static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
>  	val &= _HPAGE_CHG_MASK;
>  	val |= check_pgprot(newprot) & ~_HPAGE_CHG_MASK;
>  	val = flip_protnone_guard(oldval, val, PHYSICAL_PMD_PAGE_MASK);
> +
> +	if (pmd_dirty(pmd)) {
> +		if (static_cpu_has(X86_FEATURE_SHSTK) && !(val & _PAGE_RW))
> +			val |= _PAGE_DIRTY_SW;
> +		else
> +			val |= _PAGE_DIRTY_HW;
> +	}
> +
>  	return __pmd(val);
>  }
>  
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 826823df917f..e7e28bf7e919 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -150,8 +150,8 @@
>   * 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_HW |	\
> +#define _PAGE_CHG_MASK	(PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |	\
> +			 _PAGE_SPECIAL | _PAGE_ACCESSED |	\
>  			 _PAGE_SOFT_DIRTY | _PAGE_DEVMAP)
>  #define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)