linux-kernel - Re: [PATCH v4 10/39] x86/mm: Introduce _PAGE

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Date:   Tue, 20 Dec 2022 22:29:13 +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 10/39] x86/mm: Introduce _PAGE_COW

On Fri, Dec 02, 2022 at 04:35:37PM -0800, Rick Edgecombe wrote:
> There are six bits left available to software in the 64-bit PTE after
> consuming a bit for _PAGE_COW. No space is consumed in 32-bit kernels
> because shadow stacks are not enabled there.
> 
> This is a prepratory patch. Changes to actually start marking _PAGE_COW

Unknown word [prepratory] in commit message.
Suggestions: ['preparatory',

> will follow once other pieces are in place.

And regardless, you don't really need this sentence at all, AFAICT.

...

> +/*
> + * Normally COW memory can result in Dirty=1,Write=0 PTs. But in the case
							^^^

PTEs.

> + * of X86_FEATURE_USER_SHSTK, the software COW bit is used, since the
> + * Dirty=1,Write=0 will result in the memory being treated as shaodw stack
> + * by the HW. So when creating COW memory, a software bit is used
> + * _PAGE_BIT_COW. The following functions pte_mkcow() and pte_clear_cow()
> + * take a PTE marked conventially COW (Dirty=1) and transition it to the

Unknown word [conventially] in comment.
Suggestions: ['conventionally', ...

> + * shadow stack compatible version of COW (Cow=1).
> + */
> +

^ Superfluous newline.

> +static inline pte_t pte_mkcow(pte_t pte)
> +{
> +	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
> +		return pte;
> +
> +	pte = pte_clear_flags(pte, _PAGE_DIRTY);
> +	return pte_set_flags(pte, _PAGE_COW);
> +}
> +
> +static inline pte_t pte_clear_cow(pte_t pte)
> +{
> +	/*
> +	 * _PAGE_COW is unnecessary on !X86_FEATURE_USER_SHSTK kernels.

I'm guessing this "unnecessary" is supposed to mean that on kernels not
supporting shadow stack, a COW page uses the old bit flags?

I.e., Dirty=1,Write=0?

Might as well write it this way to be perfectly clear.

> +	 * See the _PAGE_COW definition for more details.
> +	 */
> +	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
> +		return pte;
> +
> +	/*
> +	 * PTE is getting copied-on-write, so it will be dirtied
> +	 * if writable, or made shadow stack if shadow stack and
> +	 * being copied on access. Set they dirty bit for both

"Set the dirty bit.."

> +	 * cases.
> +	 */
> +	pte = pte_set_flags(pte, _PAGE_DIRTY);
> +	return pte_clear_flags(pte, _PAGE_COW);
> +}

Rest looks ok.

Thx.

-- 
Regards/Gruss,
    Boris.

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