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

[David Hildenbrand]

On 27.03.23 20:34, Mike Kravetz wrote:
> On 03/26/23 10:46, Peter Xu wrote:
> > 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.
>
> I am OK with separate patches, and agree that the R/O pinning case is less
> likely to happen.

Yes, the combination should be rather rare and we can fix that 
separately. Ideally, we'd try to mimic the same uffd code flow in 
hugetlb cow/unshare handling that we use in memory.c

> Since this patch addresses the issue found by Muhammad,
>
> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Hopefully we didn't forget about yet another case :D

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb