[PATCH] mm/util: don't read __page_2 for order-1 folios in snapshot_page()

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

snapshot_page() reconstructs a folio from a struct page.  After copying
the head and __page_1 it reads __page_2 whenever the folio has more than
one page:

	if (nr_pages > 1)
		memcpy(&ps->folio_snapshot.__page_2, &foliop->__page_2,
		       sizeof(struct page));

__page_2 is the folio's third struct page, so it is part of the folio
only for order >= 2 (nr_pages > 2).  For an order-1 folio (exactly two
pages) __page_2 is not part of the folio at all, it is the struct page
of the following pfn.

When such an order-1 head sits in the last struct page slots of a
populated section whose neighbouring section is absent (a memory hole),
__page_2 falls into the next section's unpopulated vmemmap and the
read oopses.

Observed on a 22 TB ppc64le LPAR during DLPAR memory remove, on the page
isolation dump path:

	offline_pages -> start_isolate_page_range -> isolate_single_pageblock
	  -> set_migratetype_isolate -> dump_page -> __dump_page -> snapshot_page

	NIP   = snapshot_page+264  (ld of __page_2)
	r4    = foliop = head = 0xc00c0005a03fff80
	DAR   = r4 + 0x88     = 0xc00c0005a0400008   (unmapped)
	DSISR = 0x40000000                           (no translation)

The faulting head was a free page that still carried PG_head with
_nr_pages == 2; its __page_2 is the first entry of the absent section.

It is also reproducible deterministically in a VM by placing an order-1
folio in the last slots of a populated section adjacent to a hole
(memmap=nnM$ssM) and calling dump_page() on it.

Only read __page_2 for order >= 2 folios (nr_pages > 2).

Fixes: 31a31da8a618 ("mm: move _pincount in folio to page[2] on 32bit")
Cc: stable@vger.kernel.org # v6.15+
Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
---
 mm/util.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index af2c2103f0d95..b3d48a05e6d82 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1353,7 +1353,13 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
 	if (ps->idx < MAX_FOLIO_NR_PAGES) {
 		memcpy(&ps->folio_snapshot, foliop, 2 * sizeof(struct page));
 		nr_pages = folio_nr_pages(&ps->folio_snapshot);
-		if (nr_pages > 1)
+		/*
+		 * __page_2 is the folio's third struct page and is part of the
+		 * folio only for order >= 2 (nr_pages > 2).  For an order-1
+		 * folio it is not part of the folio and may fall into an
+		 * adjacent, possibly absent, section.
+		 */
+		if (nr_pages > 2)
 			memcpy(&ps->folio_snapshot.__page_2, &foliop->__page_2,
 			       sizeof(struct page));
 		set_ps_flags(ps, foliop, page);
-- 
2.54.0