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@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Luboslav Pivarc <lpivarc@xxxxxxxxxx>
> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx>
--
Thanks,
David / dhildenb
