linux-kernel - Re: [PATCH 6/9] x86/mm: Introduce ptep_set_wrprotect

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Date:   Thu, 7 Jun 2018 09:24:03 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. J. Lu" <hjl.tools@...il.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        mike.kravetz@...cle.com
Subject: Re: [PATCH 6/9] x86/mm: Introduce ptep_set_wrprotect_flush and
 related functions

On Thu, Jun 7, 2018 at 7:40 AM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
>
> The function ptep_set_wrprotect()/huge_ptep_set_wrprotect() is
> used by copy_page_range()/copy_hugetlb_page_range() to copy
> PTEs.
>
> On x86, when the shadow stack is enabled, only a shadow stack
> PTE has the read-only and _PAGE_DIRTY_HW combination.  Upon
> making a dirty PTE read-only, we move its _PAGE_DIRTY_HW to
> _PAGE_DIRTY_SW.
>
> When ptep_set_wrprotect() moves _PAGE_DIRTY_HW to _PAGE_DIRTY_SW,
> if the PTE is writable and the mm is shared, another task could
> race to set _PAGE_DIRTY_HW again.
>
> Introduce ptep_set_wrprotect_flush(), pmdp_set_wrprotect_flush(),
> and huge_ptep_set_wrprotect_flush() to make sure this does not
> happen.
>

This patch adds flushes where they didn't previously exist.

> +static inline void ptep_set_wrprotect_flush(struct vm_area_struct *vma,
> +                                           unsigned long addr, pte_t *ptep)
> +{
> +       bool rw;
> +
> +       rw = test_and_clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
> +       if (IS_ENABLED(CONFIG_X86_INTEL_SHADOW_STACK_USER)) {
> +               struct mm_struct *mm = vma->vm_mm;
> +               pte_t pte;
> +
> +               if (rw && (atomic_read(&mm->mm_users) > 1))
> +                       pte = ptep_clear_flush(vma, addr, ptep);

Why are you clearing the pte?

> -#define __HAVE_ARCH_PMDP_SET_WRPROTECT
> -static inline void pmdp_set_wrprotect(struct mm_struct *mm,
> -                                     unsigned long addr, pmd_t *pmdp)
> +#define __HAVE_ARCH_HUGE_PTEP_SET_WRPROTECT_FLUSH
> +static inline void huge_ptep_set_wrprotect_flush(struct vm_area_struct *vma,
> +                                                unsigned long addr, pte_t *ptep)
>  {
> -       clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp);
> +       ptep_set_wrprotect_flush(vma, addr, ptep);

Maybe I'm just missing something, but you're changed the semantics of
this function significantly.