Re: [PATCH v9 11/42] x86/mm: Update ptep/pmdp_set_wrprotect() for _PAGE_SAVED_DIRTY

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Mon, Jun 12, 2023 at 5:14 PM Rick Edgecombe
<rick.p.edgecombe@xxxxxxxxx> 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