Commit e32d20c0 authored by Mike Kravetz's avatar Mike Kravetz Committed by Linus Torvalds
Browse files

hugetlb: before freeing hugetlb page set dtor to appropriate value 

When removing a hugetlb page from the pool the ref count is set to one (as
the free page has no ref count) and compound page destructor is set to
NULL_COMPOUND_DTOR.  Since a subsequent call to free the hugetlb page will
call __free_pages for non-gigantic pages and free_gigantic_page for
gigantic pages the destructor is not used.

However, consider the following race with code taking a speculative
reference on the page:

Thread 0				Thread 1
--------				--------
remove_hugetlb_page
  set_page_refcounted(page);
  set_compound_page_dtor(page,
           NULL_COMPOUND_DTOR);
					get_page_unless_zero(page)
__update_and_free_page
  __free_pages(page,
           huge_page_order(h));

		/* Note that __free_pages() will simply drop
		   the reference to the page. */

					put_page(page)
					  __put_compound_page()
					    destroy_compound_page
					      NULL_COMPOUND_DTOR
						BUG: kernel NULL pointer
						dereference, address:
						0000000000000000

To address this race, set the dtor to the normal compound page dtor for
non-gigantic pages.  The dtor for gigantic pages does not matter as
gigantic pages are changed from a compound page to 'just a group of pages'
before freeing.  Hence, the destructor is not used.

Link: https://lkml.kernel.org/r/20210809184832.18342-4-mike.kravetz@oracle.com


Signed-off-by: default avatarMike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: default avatarMuchun Song <songmuchun@bytedance.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc: Mina Almasry <almasrymina@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent b65a4eda

mm/hugetlb.c

+21 −1
Original line number Diff line number Diff line
@@ -1370,8 +1370,28 @@ static void remove_hugetlb_page(struct hstate *h, struct page *page, 
		h->surplus_huge_pages_node[nid]--;

	}



	/*

	 * Very subtle

	 *

	 * For non-gigantic pages set the destructor to the normal compound

	 * page dtor.  This is needed in case someone takes an additional

	 * temporary ref to the page, and freeing is delayed until they drop

	 * their reference.

	 *

	 * For gigantic pages set the destructor to the null dtor.  This

	 * destructor will never be called.  Before freeing the gigantic

	 * page destroy_compound_gigantic_page will turn the compound page

	 * into a simple group of pages.  After this the destructor does not

	 * apply.

	 *

	 * This handles the case where more than one ref is held when and

	 * after update_and_free_page is called.

	 */

	set_page_refcounted(page);

	if (hstate_is_gigantic(h))

		set_compound_page_dtor(page, NULL_COMPOUND_DTOR);

	else

		set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);



	h->nr_huge_pages--;

	h->nr_huge_pages_node[nid]--;