Re: [PATCH v4 2/4] mm,page_owner: Fix refcount imbalance

Kees Cook
2024-05-06T14:59:18Z

On Thu, Apr 04, 2024 at 09:07:00AM +0200, Oscar Salvador wrote:
> Current code does not contemplate scenarios were an allocation and
> free operation on the same pages do not handle it in the same amount
> at once.
> To give an example, page_alloc_exact(), where we will allocate a page
> of enough order to stafisfy the size request, but we will free the
> remainings right away.
>
> In the above example, we will increment the stack_record refcount
> only once, but we will decrease it the same number of times as number
> of unused pages we have to free.
> This will lead to a warning because of refcount imbalance.
>
> Fix this by recording the number of base pages in the refcount field.
>
> Reported-by: syzbot+41bbfdb8d41003d12c0f@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-mm/00000000000090e8ff0613eda0e5@google.com
> Fixes: 217b2119b9e2 ("mm,page_owner: implement the tracking of the stacks count")

Does this also fix this?
https://lore.kernel.org/all/202405061514.23fedba1-oliver.sang@intel.com/

This is a report of the backtrace changing, but the warning was
pre-existing.

> [...]
> -static void dec_stack_record_count(depot_stack_handle_t handle)
> +static void dec_stack_record_count(depot_stack_handle_t handle,
> + int nr_base_pages)
> {
> struct stack_record *stack_record = __stack_depot_get_stack_record(handle);
>
> - if (stack_record)
> - refcount_dec(&stack_record->count);
> + if (!stack_record)
> + return;
> +
> + if (refcount_sub_and_test(nr_base_pages, &stack_record->count))
> + pr_warn("%s: refcount went to 0 for %u handle\n", __func__,
> + handle);

This pr_warn() isn't needed: refcount will very loudly say the same
thing. :)

--
Kees Cook