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

[David Hildenbrand]

On 24.03.23 23:27, Mike Kravetz wrote:
> On 03/24/23 10:26, Peter Xu wrote:
> > This patch fixes an issue that a hugetlb uffd-wr-protected mapping can be
> > writable even with uffd-wp bit set.  It only happens with hugetlb private
> > mappings, when someone firstly wr-protects a missing pte (which will
> > install a pte marker), then a write to the same page without any prior
> > access to the page.
> >
> > Userfaultfd-wp trap for hugetlb was implemented in hugetlb_fault() before
> > reaching hugetlb_wp() to avoid taking more locks that userfault won't need.
> > However there's one CoW optimization path that can trigger hugetlb_wp()
> > inside hugetlb_no_page(), which will bypass the trap.
> >
> > This patch skips hugetlb_wp() for CoW and retries the fault if uffd-wp bit
> > is detected.  The new path will only trigger in the CoW optimization path
> > because generic hugetlb_fault() (e.g. when a present pte was wr-protected)
> > will resolve the uffd-wp bit already.  Also make sure anonymous UNSHARE
> > won't be affected and can still be resolved, IOW only skip CoW not CoR.
> >
> > This patch will be needed for v5.19+ hence copy stable.
> >
> > Reported-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> > Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
> > Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> >
> > Notes:
> >
> > v2 is not on the list but in an attachment in the reply; this v3 is mostly
> > to make sure it's not the same as the patch used to be attached.  Sorry
> > Andrew, we need to drop the queued one as I rewrote the commit message.
>
> My appologies!  I saw the code path missed in v2 and assumed you did not
> think it applied.  So, I said nothing.  My bad!
>
> > Muhammad, I didn't attach your T-b because of the slight functional change.
> > Please feel free to re-attach if it still works for you (which I believe
> > should).
> >
> > thanks,
> > ---
> >   mm/hugetlb.c | 14 ++++++++++++--
> >   1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 8bfd07f4c143..a58b3739ed4b 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -5478,7 +5478,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
> >   		       struct folio *pagecache_folio, spinlock_t *ptl)
> >   {
> >   	const bool unshare = flags & FAULT_FLAG_UNSHARE;
> > -	pte_t pte;
> > +	pte_t pte = huge_ptep_get(ptep);
> >   	struct hstate *h = hstate_vma(vma);
> >   	struct page *old_page;
> >   	struct folio *new_folio;
> > @@ -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

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.

--
Thanks,

David / dhildenb