Re: [syzbot ci] mm: page_ext: add count limit to page_ext_iter_next to prevent invalid PFN access

On 22 Jun 2026, at 13:01, syzbot ci wrote:

> syzbot ci has tested the following series
>
> [v2] mm: page_ext: add count limit to page_ext_iter_next to prevent invalid PFN access
> https://lore.kernel.org/all/[email protected]
> * [PATCH v2] mm: page_ext: add count limit to page_ext_iter_next to prevent invalid PFN access
>
> and found the following issue:
> WARNING in depot_fetch_stack
>
> Full report is available here:
> https://ci.syzbot.org/series/092dd7dc-cb78-46b6-8703-6044fff2631d
>
> ***
>
> WARNING in depot_fetch_stack
>
> tree:      mm-new
> URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/akpm/mm.git
> base:      e1201ff76176ef666b13d1a4ec6b6190ddc6abc8
> arch:      amd64
> compiler:  Debian clang version 22.1.6 (++20260514074242+fc4aad7b5db3-1~exp1~20260514074407.73), Debian LLD 22.1.6
> config:    https://ci.syzbot.org/builds/18f461a2-7098-44bc-9d42-634b56ba48d9/config
>
> ------------[ cut here ]------------
> !refcount_read(&stack->count)
> WARNING: lib/stackdepot.c:517 at depot_fetch_stack+0x91/0xa0, CPU#0: kworker/u9:4/1114
> Modules linked in:
> CPU: 0 UID: 0 PID: 1114 Comm: kworker/u9:4 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> Workqueue: events_unbound call_usermodehelper_exec_work
> RIP: 0010:depot_fetch_stack+0x91/0xa0
> Code: 39 f5 72 d0 48 8d 3d 7e 1b 4d 0b 89 ee 44 89 f2 89 d9 67 48 0f b9 3a 31 c0 5b 41 5e 5d e9 87 67 b8 06 cc 90 0f 0b 90 eb ee 90 <0f> 0b 90 eb e8 66 2e 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90
> RSP: 0000:ffffc900079a6ce0 EFLAGS: 00010246
> RAX: ffff888168b94000 RBX: 0000000000000ce0 RCX: 0000000000000067
> RDX: 0000000000000000 RSI: ffffffff8e215937 RDI: ffffffff8c28ab20
> RBP: 0000000000000067 R08: ffff88810495a407 R09: 1ffff1102092b480
> R10: dffffc0000000000 R11: ffffed102092b481 R12: 00000000019c0068
> R13: 0000000000000001 R14: 000000000000010f R15: ffff88810afb1dc0
> FS:  0000000000000000(0000) GS:ffff88818dcb5000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: ffff88823ffff000 CR3: 000000000e74a000 CR4: 00000000000006f0
> Call Trace:
>  <TASK>
>  __set_page_owner+0x140/0x4c0
>  post_alloc_hook+0x1f9/0x250
>  get_page_from_freelist+0x21fa/0x2270
>  __alloc_frozen_pages_noprof+0x18d/0x380
>  alloc_pages_mpol+0x212/0x380
>  alloc_pages_noprof+0xac/0x2a0
>  get_free_pages_noprof+0xf/0x80
>  __kasan_populate_vmalloc+0x38/0x1c0
>  alloc_vmap_area+0xd1a/0x1420
>  __get_vm_area_node+0x1f2/0x300
>  __vmalloc_node_range_noprof+0x358/0x1730
>  __vmalloc_node_noprof+0xc2/0x100
>  dup_task_struct+0x28e/0x830
>  copy_process+0x79d/0x4380
>  kernel_clone+0x2d7/0x940
>  user_mode_thread+0x110/0x180
>  call_usermodehelper_exec_work+0x5c/0x230
>  process_scheduled_works+0xa8e/0x14e0
>  worker_thread+0xa47/0xfb0
>  kthread+0x389/0x470
>  ret_from_fork+0x514/0xb70
>  ret_from_fork_asm+0x1a/0x30
>  </TASK>
>
>
> ***
>
> If these findings have caused you to resend the series or submit a
> separate fix, please add the following tag to your commit message:
>   Tested-by: [email protected]
>
> ---
> This report is generated by a bot. It may contain errors.
> syzbot ci engineers can be reached at [email protected].
>
> To test a patch for this bug, please reply with `#syz test`
> (should be on a separate line).
>
> The patch should be attached to the email.
> Note: arguments like custom git repos and branches are not supported.

#syz test

Best Regards,
Yan, Zi

diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
index 4f7d7a8709de..f797bc898b8b 100644
--- a/include/linux/page_ext.h
+++ b/include/linux/page_ext.h
@@ -156,7 +156,8 @@ static inline struct page_ext *page_ext_iter_next(struct page_ext_iter *iter,
 	if (WARN_ON_ONCE(!iter->page_ext))
 		return NULL;
 
-	if (iter->index++ >= count)
+	iter->index++;
+	if (iter->index >= count)
 		return NULL;
 	pfn = iter->start_pfn + iter->index;