Re: [PATCH v2 2/4] mm: page_ext: add an iteration API for page extensions

[David Hildenbrand <david@redhat.com>]

On 24.02.25 22:59, Luiz Capitulino wrote:
> The page extension implementation assumes that all page extensions of
> a given page order are stored in the same memory section. The function
> page_ext_next() relies on this assumption by adding an offset to the
> current object to return the next adjacent page extension.
> 
> This behavior works as expected for flatmem but fails for sparsemem when
> using 1G pages. The commit cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for
> gigantic folios") exposes this issue, making it possible for a crash when
> using page_owner or page_table_check page extensions.
> 
> The problem is that for 1G pages, the page extensions may span memory
> section boundaries and be stored in different memory sections. This issue
> was not visible before commit cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP
> for gigantic folios") because alloc_contig_pages() never passed more than
> MAX_PAGE_ORDER to post_alloc_hook(). However, the series introducing
> mentioned commit changed this behavior allowing the full 1G page order
> to be passed.
> 
> Reproducer:
> 
>   1. Build the kernel with CONFIG_SPARSEMEM=y and table extensions
>      support
>   2. Pass 'default_hugepagesz=1 page_owner=on' in the kernel command-line
>   3. Reserve one 1G page at run-time, this should crash (backtrace below)
> 
> To address this issue, this commit introduces a new API for iterating
> through page extensions. The main iteration macro is for_each_page_ext()
> and it must be called with the RCU read lock taken. Here's an usage
> example:
> 
> """
> struct page_ext_iter iter;
> struct page_ext *page_ext;
> 
> ...
> 
> rcu_read_lock();
> for_each_page_ext(page, 1 << order, page_ext, iter) {
> 	struct my_page_ext *obj = get_my_page_ext_obj(page_ext);
> 	...
> }
> rcu_read_unlock();
> """
> 
> The loop construct uses page_ext_iter_next() which checks to see if we
> have crossed sections in the iteration. In this case, page_ext_iter_next()
> retrieves the next page_ext object from another section.
> 
> Thanks to David Hildenbrand for helping identify the root cause and
> providing suggestions on how to fix and optmize the solution (final
> implementation and bugs are all mine through).
> 
> Lastly, here's the backtrace, without kasan you can get random crashes:
> 
> [   76.052526] BUG: KASAN: slab-out-of-bounds in __update_page_owner_handle+0x238/0x298
> [   76.060283] Write of size 4 at addr ffff07ff96240038 by task tee/3598
> [   76.066714]
> [   76.068203] CPU: 88 UID: 0 PID: 3598 Comm: tee Kdump: loaded Not tainted 6.13.0-rep1 #3
> [   76.076202] Hardware name: WIWYNN Mt.Jade Server System B81.030Z1.0007/Mt.Jade Motherboard, BIOS 2.10.20220810 (SCP: 2.10.20220810) 2022/08/10
> [   76.088972] Call trace:
> [   76.091411]  show_stack+0x20/0x38 (C)
> [   76.095073]  dump_stack_lvl+0x80/0xf8
> [   76.098733]  print_address_description.constprop.0+0x88/0x398
> [   76.104476]  print_report+0xa8/0x278
> [   76.108041]  kasan_report+0xa8/0xf8
> [   76.111520]  __asan_report_store4_noabort+0x20/0x30
> [   76.116391]  __update_page_owner_handle+0x238/0x298
> [   76.121259]  __set_page_owner+0xdc/0x140
> [   76.125173]  post_alloc_hook+0x190/0x1d8
> [   76.129090]  alloc_contig_range_noprof+0x54c/0x890
> [   76.133874]  alloc_contig_pages_noprof+0x35c/0x4a8
> [   76.138656]  alloc_gigantic_folio.isra.0+0x2c0/0x368
> [   76.143616]  only_alloc_fresh_hugetlb_folio.isra.0+0x24/0x150
> [   76.149353]  alloc_pool_huge_folio+0x11c/0x1f8
> [   76.153787]  set_max_huge_pages+0x364/0xca8
> [   76.157961]  __nr_hugepages_store_common+0xb0/0x1a0
> [   76.162829]  nr_hugepages_store+0x108/0x118
> [   76.167003]  kobj_attr_store+0x3c/0x70
> [   76.170745]  sysfs_kf_write+0xfc/0x188
> [   76.174492]  kernfs_fop_write_iter+0x274/0x3e0
> [   76.178927]  vfs_write+0x64c/0x8e0
> [   76.182323]  ksys_write+0xf8/0x1f0
> [   76.185716]  __arm64_sys_write+0x74/0xb0
> [   76.189630]  invoke_syscall.constprop.0+0xd8/0x1e0
> [   76.194412]  do_el0_svc+0x164/0x1e0
> [   76.197891]  el0_svc+0x40/0xe0
> [   76.200939]  el0t_64_sync_handler+0x144/0x168
> [   76.205287]  el0t_64_sync+0x1ac/0x1b0
> 
> Fixes: cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for gigantic folios")
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---


Acked-by: David Hildenbrand <david@redhat.com>


-- 
Cheers,

David / dhildenb