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: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Muchun 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 8f97321396cc8bd7c9cc13b214681a7aff353622..dd1c1e7d970b767b395d6f9054f9feb9b04e2242 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -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);
-       set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
+       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]--;