Re: [PATCH 6/6] workqueue, kasan: avoid alloc_pages() when recording stack

[Tejun Heo <tj@kernel.org>]

On Tue, Sep 07, 2021 at 04:13:07PM +0200, Marco Elver wrote:
> Shuah Khan reported:
> 
>  | When CONFIG_PROVE_RAW_LOCK_NESTING=y and CONFIG_KASAN are enabled,
>  | kasan_record_aux_stack() runs into "BUG: Invalid wait context" when
>  | it tries to allocate memory attempting to acquire spinlock in page
>  | allocation code while holding workqueue pool raw_spinlock.
>  |
>  | There are several instances of this problem when block layer tries
>  | to __queue_work(). Call trace from one of these instances is below:
>  |
>  |     kblockd_mod_delayed_work_on()
>  |       mod_delayed_work_on()
>  |         __queue_delayed_work()
>  |           __queue_work() (rcu_read_lock, raw_spin_lock pool->lock held)
>  |             insert_work()
>  |               kasan_record_aux_stack()
>  |                 kasan_save_stack()
>  |                   stack_depot_save()
>  |                     alloc_pages()
>  |                       __alloc_pages()
>  |                         get_page_from_freelist()
>  |                           rm_queue()
>  |                             rm_queue_pcplist()
>  |                               local_lock_irqsave(&pagesets.lock, flags);
>  |                               [ BUG: Invalid wait context triggered ]
> 
> The default kasan_record_aux_stack() calls stack_depot_save() with
> GFP_NOWAIT, which in turn can then call alloc_pages(GFP_NOWAIT, ...).
> In general, however, it is not even possible to use either GFP_ATOMIC
> nor GFP_NOWAIT in certain non-preemptive contexts, including
> raw_spin_locks (see gfp.h and ab00db216c9c7).
> 
> Fix it by instructing stackdepot to not expand stack storage via
> alloc_pages() in case it runs out by using kasan_record_aux_stack_noalloc().
> 
> While there is an increased risk of failing to insert the stack trace,
> this is typically unlikely, especially if the same insertion had already
> succeeded previously (stack depot hit). For frequent calls from the same
> location, it therefore becomes extremely unlikely that
> kasan_record_aux_stack_noalloc() fails.
> 
> Link: https://lkml.kernel.org/r/20210902200134.25603-1-skhan@linuxfoundation.org
> Reported-by: Shuah Khan <skhan@linuxfoundation.org>
> Signed-off-by: Marco Elver <elver@google.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun