linux-kernel - Re: [PATCH] vfio/type1: Unpin zero pages

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Message-ID: <20220830091110.3f6d1737.alex.williamson@redhat.com>
Date:   Tue, 30 Aug 2022 09:11:10 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        lpivarc@...hat.com
Subject: Re: [PATCH] vfio/type1: Unpin zero pages

On Tue, 30 Aug 2022 09:59:33 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 30.08.22 05:05, Alex Williamson wrote:
> > There's currently a reference count leak on the zero page.  We increment
> > the reference via pin_user_pages_remote(), but the page is later handled
> > as an invalid/reserved page, therefore it's not accounted against the
> > user and not unpinned by our put_pfn().
> > 
> > Introducing special zero page handling in put_pfn() would resolve the
> > leak, but without accounting of the zero page, a single user could
> > still create enough mappings to generate a reference count overflow.
> > 
> > The zero page is always resident, so for our purposes there's no reason
> > to keep it pinned.  Therefore, add a loop to walk pages returned from
> > pin_user_pages_remote() and unpin any zero pages.
> > 
> > Cc: David Hildenbrand <david@...hat.com>
> > Cc: stable@...r.kernel.org
> > Reported-by: Luboslav Pivarc <lpivarc@...hat.com>
> > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > ---
> >  drivers/vfio/vfio_iommu_type1.c |   12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index db516c90a977..8706482665d1 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -558,6 +558,18 @@ static int vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr,
> >  	ret = pin_user_pages_remote(mm, vaddr, npages, flags | FOLL_LONGTERM,
> >  				    pages, NULL, NULL);
> >  	if (ret > 0) {
> > +		int i;
> > +
> > +		/*
> > +		 * The zero page is always resident, we don't need to pin it
> > +		 * and it falls into our invalid/reserved test so we don't
> > +		 * unpin in put_pfn().  Unpin all zero pages in the batch here.
> > +		 */
> > +		for (i = 0 ; i < ret; i++) {
> > +			if (unlikely(is_zero_pfn(page_to_pfn(pages[i]))))
> > +				unpin_user_page(pages[i]);
> > +		}
> > +
> >  		*pfn = page_to_pfn(pages[0]);
> >  		goto done;
> >  	}
> > 
> >   
> 
> As discussed offline, for the shared zeropage (that's not even
> refcounted when mapped into a process), this makes perfect sense to me.
> 
> Good question raised by Sean if ZONE_DEVICE pages might similarly be
> problematic. But for them, we cannot simply always unpin here.

What sort of VM mapping would give me ZONE_DEVICE pages?  Thanks,

Alex