[PATCH v2 1/2] mm/page_isolation: fix UBSAN shift-out-of-bounds warning

[Qi Xi]

A contig-range allocation racing with buddy allocation on the adjacent
pageblock can trigger:

 UBSAN: shift-out-of-bounds in mm/page_isolation.c:393:15
 shift exponent -749042176 is negative
 Call trace:
  isolate_single_pageblock
  start_isolate_page_range
  alloc_contig_frozen_range_noprof
  alloc_contig_range_noprof

isolate_single_pageblock() first calls set_migratetype_isolate() with
zone->lock held, which marks the pageblock MIGRATE_ISOLATE and moves any
free page straddling the boundary out of the way.  Once the lock is
dropped, it scans the MAX_ORDER_NR_PAGES-aligned window [start_pfn,
boundary_pfn) locklessly, only to skip the free pages already handled
above and to detect in-use pages straddling the boundary.  Since this
scan only reads page state to decide how far to skip and returns -EBUSY
on a straddling in-use page, it does not take the lock.

The window also covers the adjacent pageblock, whose free pages stay on
the normal movable/CMA freelist and can be allocated concurrently.  So
after the scan observes PageBuddy(page), another CPU can allocate the
page, leaving a stale value in page->private that makes "1 << order" shift
out of range.

Use buddy_order_unsafe() with READ_ONCE to read the order, and validate
it is within MAX_PAGE_ORDER before shifting to prevent UBSAN warnings.

Since pageblock_isolate_and_move_free_pages() already handles free pages
straddling boundary_pfn under zone->lock, bail out with -EBUSY instead
of VM_WARN_ON_ONCE() when a PageBuddy page appears to cross the boundary
during the lockless scan.

Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granularity")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Qi Xi <xiqi2@xxxxxxxxxx>
---
 mm/page_isolation.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 32ce8a7d9df3..f2b648a68531 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -387,13 +387,15 @@ static int isolate_single_pageblock(unsigned long boundary_pfn,
 		}
 
 		if (PageBuddy(page)) {
-			int order = buddy_order(page);
+			unsigned int order = buddy_order_unsafe(page);
 
-			/* pageblock_isolate_and_move_free_pages() handled this */
-			VM_WARN_ON_ONCE(pfn + (1 << order) > boundary_pfn);
-
-			pfn += 1UL << order;
-			continue;
+			/* buddy_order_unsafe() is racy. Validate the order before shifting. */
+			if (order <= MAX_PAGE_ORDER &&
+			    pfn + (1UL << order) <= boundary_pfn) {
+				pfn += 1UL << order;
+				continue;
+			}
+			goto failed;
 		}
 
 		/*
-- 
2.33.0