linux-kernel - Re: [PATCH v9 11/42] x86/mm: Update ptep/pmdp_set_wrprotect() for _PAGE_SAVED

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Message-ID: <CAHk-=wh-PfXhVb7Atk+G8SNjFSvqDWu37KPFR-ssuM-j_o93Kg@mail.gmail.com>
Date:   Tue, 13 Jun 2023 11:01:02 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.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>,
        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,
        david@...hat.com, debug@...osinc.com, szabolcs.nagy@....com,
        broonie@...nel.org, Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v9 11/42] x86/mm: Update ptep/pmdp_set_wrprotect() for _PAGE_SAVED_DIRTY

On Mon, Jun 12, 2023 at 5:14 PM Rick Edgecombe
<rick.p.edgecombe@...el.com> wrote:
>
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1189,7 +1189,17 @@ static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
>  static inline void ptep_set_wrprotect(struct mm_struct *mm,
>                                       unsigned long addr, pte_t *ptep)
>  {
> -       clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
> +       /*
> +        * Avoid accidentally creating shadow stack PTEs
> +        * (Write=0,Dirty=1).  Use cmpxchg() to prevent races with
> +        * the hardware setting Dirty=1.
> +        */
> +       pte_t old_pte, new_pte;
> +
> +       old_pte = READ_ONCE(*ptep);
> +       do {
> +               new_pte = pte_wrprotect(old_pte);
> +       } while (!try_cmpxchg((long *)&ptep->pte, (long *)&old_pte, *(long *)&new_pte));
>  }

Thanks. Much nicer with this all being done just one way and no need
for ifdeffery on config options and runtime static branches.

                  Linus