Hello Zi Yan,
Commit fc4d182316bd ("mm: huge_memory: enable debugfs to split huge
pages to any order") from Feb 26, 2024 (linux-next), leads to the
following Smatch static checker warning:
mm/huge_memory.c:2898 __split_huge_page()
error: undefined (user controlled) shift '1 << new_order'
mm/huge_memory.c
2889 static void __split_huge_page(struct page *page, struct list_head *list,
2890 pgoff_t end, unsigned int new_order)
2891 {
2892 struct folio *folio = page_folio(page);
2893 struct page *head = &folio->page;
2894 struct lruvec *lruvec;
2895 struct address_space *swap_cache = NULL;
2896 unsigned long offset = 0;
2897 int i, nr_dropped = 0;
--> 2898 unsigned int new_nr = 1 << new_order;
^^^^^^^^^
The new_order variable comes from the user via debugfs.
2899 int order = folio_order(folio);
2900 unsigned int nr = 1 << order;
2901
2902 /* complete memcg works before add pages to LRU */
2903 split_page_memcg(head, order, new_order);
2904
2905 if (folio_test_anon(folio) && folio_test_swapcache(folio)) {
2906 offset = swp_offset(folio->swap);
2907 swap_cache = swap_address_space(folio->swap);
Here is the debugfs code in split_huge_pages_write()
mm/huge_memory.c
3628
3629 ret = sscanf(input_buf, "%d,0x%lx,0x%lx,%d", &pid, &vaddr_start, &vaddr_end, &new_order);
^^^^^^^^^^
We just read new_order
3630 if (ret == 1 && pid == 1) {
3631 split_huge_pages_all();
3632 ret = strlen(input_buf);
3633 goto out;
3634 } else if (ret != 3 && ret != 4) {
3635 ret = -EINVAL;
3636 goto out;
3637 }
3638
3639 ret = split_huge_pages_pid(pid, vaddr_start, vaddr_end, new_order);
^^^^^^^^^
And pass it directly with no bounds checking. Debugfs code is root
only... We used to take a view that if root does something stupid then
they get what they deserve. But these days syzbot is fuzz testing stuff
even when it's root only and complaining about shift wraps or other
undefined behavior. So I feel like it might be easiest to silence this
undefined behavior warning now instead of waiting for the syzbot reports
to come back to bite us in a couple years.
3640 if (!ret)
3641 ret = strlen(input_buf);
3642 out:
3643 mutex_unlock(&split_debug_mutex);
3644 return ret;
3645
3646 }
regards,
dan carpenter
