Re: [PATCH v3] mm/hugetlb: Fix uffd wr-protection for CoW optimization path

[Peter Xu <peterx@redhat.com>]

On Fri, Mar 24, 2023 at 11:36:53PM +0100, David Hildenbrand wrote:
> > > @@ -5487,6 +5487,17 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
> > >   	unsigned long haddr = address & huge_page_mask(h);
> > >   	struct mmu_notifier_range range;
> > > +	/*
> > > +	 * Never handle CoW for uffd-wp protected pages.  It should be only
> > > +	 * handled when the uffd-wp protection is removed.
> > > +	 *
> > > +	 * Note that only the CoW optimization path (in hugetlb_no_page())
> > > +	 * can trigger this, because hugetlb_fault() will always resolve
> > > +	 * uffd-wp bit first.
> > > +	 */
> > > +	if (!unshare && huge_pte_uffd_wp(pte))
> > > +		return 0;
> > 
> > This looks correct.  However, since the previous version looked correct I must
> > ask.  Can we have unshare set and huge_pte_uffd_wp true?  If so, then it seems
> > we would need to possibly propogate that uffd_wp to the new pte as in v2

Good point, thanks for spotting!

> 
> We can. A reproducer would share an anon hugetlb page because parent and
> child. In the parent, we would uffd-wp that page. We could trigger unsharing
> by R/O-pinning that page.

Right.  This seems to be a separate bug..  It should be triggered in
totally different context and much harder due to rare use of RO pins,
meanwhile used with userfault-wp.

If both of you agree, I can prepare a separate patch for this bug, and I'll
better prepare a reproducer/selftest with it.

-- 
Peter Xu