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

Borislav Petkov
2022-12-27T11:42:32Z

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.