[PATCH v2 0/2] mm/page_isolation: fix UBSAN shift-out-of-bounds in isolate_single_pageblock

[PATCH v2 0/2] mm/page_isolation: fix UBSAN shift-out-of-bounds in isolate_single_pageblock

[Qi Xi]

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.

Changes since v1:
- Patch 1: drop VM_WARN_ON_ONCE, bail out with -EBUSY instead
- Patch 2: new patch addressing PageCompound branch race


Qi Xi (2):
  mm/page_isolation: fix UBSAN shift-out-of-bounds warning
  mm/page_isolation: guard compound_order() against racing

 mm/page_isolation.c | 36 ++++++++++++++++++++++++++++--------
 1 file changed, 28 insertions(+), 8 deletions(-)

-- 
2.33.0

[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@vger.kernel.org
Signed-off-by: Qi Xi <xiqi2@huawei.com>
---
 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

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

[Zi Yan]

On Thu Aug 20, 2026 at 10:55 PM EDT, Qi Xi wrote:
> 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@vger.kernel.org
> Signed-off-by: Qi Xi <xiqi2@huawei.com>
> ---
>  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 &&

Can you add a comment like below to clarify the check? Thanks.
				/*
				 * pageblock_isolate_and_move_free_pages() splits
				 * cross-boundary PageBuddy, verify it.
				 */
> +			    pfn + (1UL << order) <= boundary_pfn) {
> +				pfn += 1UL << order;
> +				continue;
> +			}
> +			goto failed;
>  		}
>  
>  		/*


With that, feel free to add

Reviewed-by: Zi Yan <ziy@nvidia.com>


-- 
Best Regards,
Yan, Zi

[PATCH v2 2/2] mm/page_isolation: guard compound_order() against racing

[Qi Xi]

The PageCompound branch reads compound_head() without holding a reference.
A racing split or free can cause compound_head() to return a stale pointer,
and compound_nr() reads the order from that stale head, leading to
out-of-range shifts and making the skip distance meaningless.

Read the order explicitly with compound_order() and validate it is within
MAX_FOLIO_ORDER before shifting. Also verify the derived head_pfn against
the legitimate pfn: the head must not be past pfn, must be aligned to
nr_pages, and pfn must fall within the compound page. Bail out with
-EBUSY if any check fails.

Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granularity")
Cc: stable@vger.kernel.org
Suggested-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Qi Xi <xiqi2@huawei.com>
---
 mm/page_isolation.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index f2b648a68531..9cf7f92011bd 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -414,10 +414,28 @@ static int isolate_single_pageblock(unsigned long boundary_pfn,
 		if (PageCompound(page)) {
 			struct page *head = compound_head(page);
 			unsigned long head_pfn = page_to_pfn(head);
-			unsigned long nr_pages = compound_nr(head);
+			unsigned int order = compound_order(head);
+			unsigned long nr_pages;
+
+			/* compound_order() is racy. Cap it at MAX_FOLIO_ORDER. */
+			if (order > MAX_FOLIO_ORDER)
+				goto failed;
+
+			nr_pages = 1UL << order;
+
+			/*
+			 * compound_head() is also racy, so the derived head_pfn
+			 * needs additional checks to make sure it is valid.
+			 * Otherwise, just fail the check. pfn comes from
+			 * __first_valid_page() as a legitimate PFN, so use it to
+			 * check head_pfn.
+			 */
+			if (head_pfn > pfn || !IS_ALIGNED(head_pfn, nr_pages) ||
+			    pfn - head_pfn >= nr_pages)
+				goto failed;
 
 			if (head_pfn + nr_pages <= boundary_pfn ||
-			    PageHuge(page)) {
+			    PageHuge(head)) {
 				pfn = head_pfn + nr_pages;
 				continue;
 			}
-- 
2.33.0

Re: [PATCH v2 2/2] mm/page_isolation: guard compound_order() against racing

[Zi Yan]

On Thu Aug 20, 2026 at 10:55 PM EDT, Qi Xi wrote:
> The PageCompound branch reads compound_head() without holding a reference.
> A racing split or free can cause compound_head() to return a stale pointer,
> and compound_nr() reads the order from that stale head, leading to
> out-of-range shifts and making the skip distance meaningless.
>
> Read the order explicitly with compound_order() and validate it is within
> MAX_FOLIO_ORDER before shifting. Also verify the derived head_pfn against
> the legitimate pfn: the head must not be past pfn, must be aligned to
> nr_pages, and pfn must fall within the compound page. Bail out with
> -EBUSY if any check fails.
>
> Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock granularity")
> Cc: stable@vger.kernel.org
> Suggested-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Qi Xi <xiqi2@huawei.com>
> ---
>  mm/page_isolation.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
>
LGTM.

Reviewed-by: Zi Yan <ziy@nvidia.com>

-- 
Best Regards,
Yan, Zi