[PATCH v5 8/9] mm/page_owner: clamp skip_buddy_pages() PFN advance at MAX_ORDER_NR_PAGES boundary

[Ye Liu]

The lockless buddy_order_unsafe() read can return a garbage order
value if the page is concurrently allocated between the PageBuddy
check and the private read.  If this bogus order is <= MAX_PAGE_ORDER,
skip_buddy_pages() would arbitrarily advance the PFN, potentially
jumping past a MAX_ORDER_NR_PAGES boundary whose pfn_valid() check
would have caught an offline memory section.

In read_page_owner(), which relies solely on boundary-aligned
pfn_valid() to guard pfn_to_page(), skipping the boundary could
cause pfn_to_page() to access an unmapped mem_section.

Clamp the advance so it never crosses the next MAX_ORDER_NR_PAGES
boundary.  This is safe for all three callers: the pageblock-iterating
ones already handle boundary transitions in their outer loops, and
for read_page_owner() the worst case is one extra PageBuddy check per
1024 pages for a huge buddy block straddling the boundary.

Signed-off-by: Ye Liu <ye.liu@xxxxxxxxx>
---
 mm/page_owner.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 46a933f9c229..2e3880053a34 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -428,6 +428,12 @@ void __folio_copy_owner(struct folio *newfolio, struct folio *old)
  * to skip less than the full buddy block, but that is acceptable for page owner
  * iteration purposes.
  *
+ * The lockless read of buddy_order_unsafe() can also return a garbage order if
+ * the page is concurrently allocated and PageBuddy is cleared between the check
+ * and the read. Clamp the advance at the next MAX_ORDER_NR_PAGES boundary so
+ * that a bogus order cannot carry @pfn into an unvalidated memory section,
+ * which would break callers that rely on boundary-aligned pfn_valid() checks.
+ *
  * Return: true if the page was skipped (caller should continue its loop),
  *         false if the page is not a buddy page and should be processed normally.
  */
@@ -439,8 +445,12 @@ static inline bool skip_buddy_pages(unsigned long *pfn, struct page *page)
 		return false;
 
 	order = buddy_order_unsafe(page);
-	if (order <= MAX_PAGE_ORDER)
-		*pfn += (1UL << order) - 1;
+	if (order <= MAX_PAGE_ORDER) {
+		unsigned long new_pfn = *pfn + (1UL << order);
+		unsigned long boundary = ALIGN(*pfn + 1, MAX_ORDER_NR_PAGES);
+
+		*pfn = min(new_pfn, boundary) - 1;
+	}
 
 	return true;
 }
-- 
2.43.0