Re: [PATCH v2 12/39] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW

Kirill A . Shutemov
2022-10-03T18:03:41Z

On Thu, Sep 29, 2022 at 03:29:09PM -0700, Rick Edgecombe wrote:
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 2f2963429f48..58c7bf9d7392 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1287,6 +1287,23 @@ 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)
> {
> +#ifdef CONFIG_X86_SHADOW_STACK
> + /*
> + * Avoid accidentally creating shadow stack PTEs
> + * (Write=0,Dirty=1). Use cmpxchg() to prevent races with
> + * the hardware setting Dirty=1.
> + */
> + if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> + pte_t old_pte, new_pte;
> +
> + old_pte = READ_ONCE(*ptep);
> + do {
> + new_pte = pte_wrprotect(old_pte);
> + } while (!try_cmpxchg(&ptep->pte, &old_pte.pte, new_pte.pte));
> +
> + return;
> + }
> +#endif
> clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
> }

Okay, this addresses my previous question. The need in cmpxchg is
unfortunate, but well.

--
Kiryl Shutsemau / Kirill A. Shutemov