vfio/type1: Unpin zero pages

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: stable@vger.kernel.org
Reported-by: Luboslav Pivarc <lpivarc@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/166182871735.3518559.8884121293045337358.stgit@omen
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

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;
 	}