linux-kernel - Re: [PATCH v2 16/39] x86/mm: Update maybe

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Message-ID: <Y0mBFjfOAA+hJcUn@hirez.programming.kicks-ass.net>
Date:   Fri, 14 Oct 2022 17:32:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
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>,
        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>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com, rppt@...nel.org,
        jamorris@...ux.microsoft.com, dethoma@...rosoft.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 16/39] x86/mm: Update maybe_mkwrite() for shadow stack

On Thu, Sep 29, 2022 at 03:29:13PM -0700, Rick Edgecombe wrote:

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8cd413c5a329..fef14ab3abcb 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -981,13 +981,25 @@ void free_compound_page(struct page *page);
>   * servicing faults for write access.  In the normal case, do always want
>   * pte_mkwrite.  But get_user_pages can cause write faults for mappings
>   * that do not have writing enabled, when used by access_process_vm.
> + *
> + * If a vma is shadow stack (a type of writable memory), mark the pte shadow
> + * stack.
>   */
> +#ifndef maybe_mkwrite
>  static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
>  {
> -	if (likely(vma->vm_flags & VM_WRITE))
> +	if (!(vma->vm_flags & VM_WRITE))
> +		goto out;
> +
> +	if (vma->vm_flags & VM_SHADOW_STACK)
> +		pte = pte_mkwrite_shstk(pte);
> +	else
>  		pte = pte_mkwrite(pte);
> +
> +out:
>  	return pte;
>  }
> +#endif

Why the #ifndef guard? There is no other implementation, nor does this
patch introduce one.

Also, wouldn't it be simpler to write it like:

static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
{
	if (!(vma->vm_flags & VM_WRITE))
		return pte;

	if (vma->vm_flags & VM_SHADOW_STACK)
		return pte_mkwrite_shstk(pte);

	return pte_mkwrite(pte);
}

? (idem for the pmd version etc..)