[PATCH 6.1.y] mm/page_isolation: fix UBSAN shift-out-of-bounds warning

[Sasha Levin <sashal@kernel.org>]

From: Qi Xi <xiqi2@huawei.com>

[ Upstream commit eba0e6297edd60a85bc96ff5cfb734d066af62dd ]

Patch series "mm/page_isolation: fix UBSAN shift-out-of-bounds in
isolate_single_pageblock", v3.

Patch 1 fixes a UBSAN shift-out-of-bounds warning in the PageBuddy branch
of isolate_single_pageblock() triggered by concurrent buddy allocation.

Patch 2 addresses the same class of issue in the PageCompound branch,
where racy compound_order() and compound_head() reads could lead to
out-of-range shifts or incorrect page skipping.

This patch (of 2):

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.

Link: https://lore.kernel.org/20260825120549.966271-2-xiqi2@huawei.com
Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granularity")
Signed-off-by: Qi Xi <xiqi2@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: <stable@vger.kernel.org>
[ sashal: adapted the order check to use MAX_ORDER while preserving existing boundary-split handling ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/page_isolation.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 47fbc1696466f..bf098a6d863c0 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -372,7 +372,11 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, int flags,
 		 * always be in the range.
 		 */
 		if (PageBuddy(page)) {
-			int order = buddy_order(page);
+			unsigned int order = buddy_order_unsafe(page);
+
+			/* buddy_order_unsafe() is racy. Validate the order before shifting. */
+			if (order > MAX_ORDER)
+				goto failed;
 
 			if (pfn + (1UL << order) > boundary_pfn) {
 				/* free page changed before split, check it again */
-- 
2.53.0