Re: [PATCH v21 08/26] x86/mm: Introduce _PAGE_COW

[Borislav Petkov]

On Wed, Feb 17, 2021 at 02:27:12PM -0800, Yu-cheng Yu wrote:
> @@ -182,7 +206,7 @@ static inline int pud_young(pud_t pud)
>  
>  static inline int pte_write(pte_t pte)
>  {
> -	return pte_flags(pte) & _PAGE_RW;

Put here a comment along the lines of:

	/*
	 * Shadow stack pages are always writable - but not by normal
	 * instructions but only by shadow stack operations. Therefore, the
	 * W=0, D=1 test.
	 */

to make it clear what this means.

> +	return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);
>  }
>  
>  static inline int pte_huge(pte_t pte)
> @@ -314,6 +338,24 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
>  	return native_make_pte(v & ~clear);
>  }
>  
> +static inline pte_t pte_make_cow(pte_t pte)

pte_mkcow like the rest of the "pte_mkX" functions.

And below too, for the other newly added pXd_make_* helpers.


>  static inline pmd_t pmd_mkdirty(pmd_t pmd)
>  {
> -	return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> +	pmdval_t dirty = _PAGE_DIRTY;
> +
> +	/* Avoid creating (HW)Dirty=1, Write=0 PMDs */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pmd_flags(pmd) & _PAGE_RW))

						      !(pmd_write(pmd))

> +		dirty = _PAGE_COW;
> +
> +	return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
> +}

...

>  static inline pud_t pud_mkdirty(pud_t pud)
>  {
> -	return pud_set_flags(pud, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> +	pudval_t dirty = _PAGE_DIRTY;
> +
> +	/* Avoid creating (HW)Dirty=1, Write=0 PUDs */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pud_flags(pud) & _PAGE_RW))

						      !(pud_write(pud))


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette