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

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Date:   Tue, 30 Aug 2022 09:59:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        lpivarc@...hat.com
Subject: Re: [PATCH] vfio/type1: Unpin zero pages

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.

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb