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

[Edgecombe, Rick P]

On Wed, 2023-01-04 at 14:25 +0100, Borislav Petkov wrote:
> On Tue, Dec 27, 2022 at 11:31:37PM +0000, Edgecombe, Rick P wrote:
> > The comment is referring to the dirty bits possibly coming from
> > newprot,
> 
> Ah right, ofc.
> 
> > but looking at it now I think the code was broken trying to
> > fix the recent soft dirty test breakage. Now it might lose pre-
> > existing
> > dirty bits in the pte unessarily... I think.
> 
> Right, does this code need to be simplified?
> 
> I.e., match the shadow stack PTE (Write=0,Dirty=1) and handle that in
> a separate
> helper?
> 
> So that the flows are separate. I'm not a mm guy but this function
> makes my head
> hurt - dunno about other folks. :)

Yea, the whole Write=0,Dirty=1 thing has been a bit of a challenge to
make clear in the MM code. Dave had suggested a sketch here for
pte_modify():

https://lore.kernel.org/lkml/95299e90-245b-61c5-8ef0-5e6da3c37c5e@xxxxxxxxx/

The problem was that pte_mkdirty() also sets the soft dirty bit. So it
did more than preserve the dirty bit - it also added on the soft dirty
bit. I extracted a helper __pte_mkdirty() that can optionally not set
the soft dirty bit. So then it looks pretty close to how Dave
suggested:

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_no_dirty;
	val |= check_pgprot(newprot) & ~_page_chg_mask_no_dirty;
	val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);

	pte_result = __pte(val);

	/*
	 * Dirty bit is not preserved above so it can be done
	 * in a special way for the shadow stack case, where it
	 * may need to set _PAGE_COW. __pte_mkdirty() will do this in
	 * the case of shadow stack.
	 */
	if (pte_dirty(pte))
		pte_result = __pte_mkdirty(pte_result, false);

	return pte_result;
}