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 Suggested-by: Matthew Wilcox Signed-off-by: Ketan Kishore 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 To: David Hildenbrand To: Lorenzo Stoakes To: "Liam R. Howlett" To: Vlastimil Babka To: Mike Rapoport To: Suren Baghdasaryan To: Michal Hocko To: Luiz Capitulino 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