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

From: Mike Kravetz
Date: Mon Mar 27 2023 - 14:35:05 EST


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.

Since this patch addresses the issue found by Muhammad,

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
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Mike Kravetz