Re: [PATCH v4 11/39] x86/mm: Update pte_modify for _PAGE_COW

[Borislav Petkov]

On Fri, Dec 02, 2022 at 04:35:38PM -0800, Rick Edgecombe wrote:
>  static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
>  {
> +	pteval_t _page_chg_mask_no_dirty = _PAGE_CHG_MASK & ~_PAGE_DIRTY;
>  	pteval_t val = pte_val(pte), oldval = val;
> +	pte_t pte_result;
>  
>  	/*
>  	 * Chop off the NX bit (if present), and add the NX portion of
>  	 * the newprot (if present):
>  	 */
> -	val &= _PAGE_CHG_MASK;
> -	val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
> +	val &= _page_chg_mask_no_dirty;
> +	val |= check_pgprot(newprot) & ~_page_chg_mask_no_dirty;
>  	val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);
> -	return __pte(val);
> +
> +	pte_result = __pte(val);
> +
> +	/*
> +	 * Dirty bit is not preserved above so it can be done

Just for my own understanding: are you saying here that
flip_protnone_guard() might end up setting _PAGE_DIRTY in val...

> +	 * in a special way for the shadow stack case, where it
> +	 * needs to set _PAGE_COW. pte_mkcow() will do this in
> +	 * the case of shadow stack.
> +	 */
> +	if (pte_dirty(pte_result))
> +		pte_result = pte_mkcow(pte_result);

... and in that case we need to turn it into a _PAGE_COW setting?

Thx.

-- 
Regards/Gruss,
    Boris.

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