[PATCH v3] mm: page_ext: add count limit to page_ext_iter_next to prevent invalid PFN access

[Ketan <ketan.kishore@oss.qualcomm.com>]

The page_ext iteration API does not validate if the PFN still
belongs to a valid section while advancing the iterator. When
dynamically adding memory in the hotplug path, it can lead to a
NULL pointer dereference during page_ext_lookup at the boundary
of the last valid section when iterator count equals __pgcount.

The for_each_page_ext() macro calls page_ext_iter_next() as its
loop increment. for_each_page_ext() does a
"__page_ext = page_ext_iter_next(&__iter)" at the end. This
causes page_ext_iter_next() to increment iter->index past
__pgcount and call page_ext_lookup(start_pfn + __pgcount).
During memory hotplug (online), the PFN at start_pfn + __pgcount
may belong to a section that has not yet been initialized,
causing page_ext_lookup() to trigger a NULL pointer dereference.

[   14.555124][  T846] Call trace:
[   14.555125][  T846]  lookup_page_ext+0x6c/0x108 (P)
[   14.555127][  T846]  page_ext_lookup+0x30/0x3c
[   14.555129][  T846]  __reset_page_owner+0x11c/0x260
[   14.571201][  T846]  __free_pages_ok+0x5e8/0x8e0
[   14.571204][  T846]  __free_pages_core+0x78/0xf0
[   14.571206][  T846]  generic_online_page+0x14/0x24
[   14.597782][  T846]  online_pages+0x178/0x30c
[   14.597784][  T846]  memory_block_change_state+0x284/0x32c
[   14.597787][  T846]  memory_subsys_online+0x4c/0x64
[   14.597789][  T846]  device_online+0x88/0xb0
[   14.597791][  T846]  online_memory_block+0x30/0x40
[   14.597793][  T846]  walk_memory_blocks+0xac/0xe8
[   14.597794][  T846]  add_memory_resource+0x280/0x298
[   14.656161][  T846]  add_memory+0x60/0x98

Move the iteration boundary enforcement inside the iterator
functions, so callers cannot inadvertently access beyond the
requested range.

Fixes: 9039b9096ea2 ("mm: page_owner: use new iteration API")
Cc: stable@vger.kernel.org
Suggested-by: David Hildenbrand <david@redhat.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Ketan Kishore <ketan.kishore@oss.qualcomm.com>
Tested-by: syzbot@syzkaller.appspotmail.com
---
Changes in v3:
- Fix the iter->index++ increment to pre increment(++iter->index)
- modify the (count == 0) check to (!count)
- Link to v2: https://patch.msgid.link/20260622-page_ext-v2-1-135d4cfbc42f@oss.qualcomm.com

Changes in v2:
- Incorporated comments from David and Matthew to check for invalid PFN
  in page_ext iterator rather than checking for NULL section in
  page_ext_lookup.
- Minor improvement in commit description to include the issue with
  page_ext_iter_next
- Link to v1: https://patch.msgid.link/20260617-page_ext-v1-1-37ad802b1a38@oss.qualcomm.com

To: Andrew Morton <akpm@linux-foundation.org>
To: David Hildenbrand <david@kernel.org>
To: Lorenzo Stoakes <ljs@kernel.org>
To: "Liam R. Howlett" <liam@infradead.org>
To: Vlastimil Babka <vbabka@kernel.org>
To: Mike Rapoport <rppt@kernel.org>
To: Suren Baghdasaryan <surenb@google.com>
To: Michal Hocko <mhocko@suse.com>
To: Luiz Capitulino <luizcap@redhat.com>
Cc: kernel@oss.qualcomm.com
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
 include/linux/page_ext.h | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
index 61e876e255e8..f23d4b218da0 100644
--- a/include/linux/page_ext.h
+++ b/include/linux/page_ext.h
@@ -120,14 +120,18 @@ struct page_ext_iter {
  * page_ext_iter_begin() - Prepare for iterating through page extensions.
  * @iter: page extension iterator.
  * @pfn: PFN of the page we're interested in.
+ * @count: maximum number of page extensions to return.
  *
  * Must be called with RCU read lock taken.
  *
  * Return: NULL if no page_ext exists for this page.
  */
 static inline struct page_ext *page_ext_iter_begin(struct page_ext_iter *iter,
-						unsigned long pfn)
+		unsigned long pfn, unsigned long count)
 {
+	if (!count)
+		return NULL;
+
 	iter->index = 0;
 	iter->start_pfn = pfn;
 	iter->page_ext = page_ext_lookup(pfn);
@@ -138,19 +142,22 @@ static inline struct page_ext *page_ext_iter_begin(struct page_ext_iter *iter,
 /**
  * page_ext_iter_next() - Get next page extension
  * @iter: page extension iterator.
+ * @count: maximum number of page extensions to return.
  *
  * Must be called with RCU read lock taken.
  *
  * Return: NULL if no next page_ext exists.
  */
-static inline struct page_ext *page_ext_iter_next(struct page_ext_iter *iter)
+static inline struct page_ext *page_ext_iter_next(struct page_ext_iter *iter,
+		unsigned long count)
 {
 	unsigned long pfn;
 
 	if (WARN_ON_ONCE(!iter->page_ext))
 		return NULL;
 
-	iter->index++;
+	if (++iter->index >= count)
+		return NULL;
 	pfn = iter->start_pfn + iter->index;
 
 	if (page_ext_iter_next_fast_possible(pfn))
@@ -183,9 +190,9 @@ static inline struct page_ext *page_ext_iter_get(const struct page_ext_iter *ite
  * IMPORTANT: must be called with RCU read lock taken.
  */
 #define for_each_page_ext(__page, __pgcount, __page_ext, __iter) \
-	for (__page_ext = page_ext_iter_begin(&__iter, page_to_pfn(__page));\
-		__page_ext && __iter.index < __pgcount;          \
-		__page_ext = page_ext_iter_next(&__iter))
+	for (__page_ext = page_ext_iter_begin(&__iter, page_to_pfn(__page), __pgcount); \
+		__page_ext; \
+		__page_ext = page_ext_iter_next(&__iter, __pgcount))
 
 #else /* !CONFIG_PAGE_EXTENSION */
 struct page_ext;

---
base-commit: c425609d6ac4012c8bbf01ec2e10e801b1923a7b
change-id: 20260616-page_ext-31ef555456fc

Best regards,
--  
Ketan Kishore <ketan.kishore@oss.qualcomm.com>