Re: [PATCH] kasan: avoid overflowing quarantine size on low memory systems

[Alexander Potapenko]

On Tue, Aug 2, 2016 at 12:05 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Tue, Aug 2, 2016 at 12:00 PM, Andrey Ryabinin
> <aryabinin@xxxxxxxxxxxxx> wrote:
>>
>>
>> On 08/01/2016 05:59 PM, Alexander Potapenko wrote:
>>> If the total amount of memory assigned to quarantine is less than the
>>> amount of memory assigned to per-cpu quarantines, |new_quarantine_size|
>>> may overflow. Instead, set it to zero.
>>>
>>
>> Just curious, how did find this?
>> Overflow is possible if system has more than 32 cpus per GB of memory. AFIAK this quite unusual.
>
> I was reading code for unrelated reason.
>
>>> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>>> Fixes: 55834c59098d ("mm: kasan: initial memory quarantine
>>> implementation")
>>> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
>>> ---
>>>  mm/kasan/quarantine.c | 12 ++++++++++--
>>>  1 file changed, 10 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
>>> index 65793f1..416d3b0 100644
>>> --- a/mm/kasan/quarantine.c
>>> +++ b/mm/kasan/quarantine.c
>>> @@ -196,7 +196,7 @@ void quarantine_put(struct kasan_free_meta *info, struct kmem_cache *cache)
>>>
>>>  void quarantine_reduce(void)
>>>  {
>>> -     size_t new_quarantine_size;
>>> +     size_t new_quarantine_size, percpu_quarantines;
>>>       unsigned long flags;
>>>       struct qlist_head to_free = QLIST_INIT;
>>>       size_t size_to_free = 0;
>>> @@ -214,7 +214,15 @@ void quarantine_reduce(void)
>>>        */
>>>       new_quarantine_size = (READ_ONCE(totalram_pages) << PAGE_SHIFT) /
>>>               QUARANTINE_FRACTION;
>>> -     new_quarantine_size -= QUARANTINE_PERCPU_SIZE * num_online_cpus();
>>> +     percpu_quarantines = QUARANTINE_PERCPU_SIZE * num_online_cpus();
>>> +     if (new_quarantine_size < percpu_quarantines) {
>>> +             WARN_ONCE(1,
>>> +                     "Too little memory, disabling global KASAN quarantine.\n",
>>> +             );
>>
>> Why WARN? I'd suggest pr_warn_once();
>
>
> I would suggest to just do something useful. Setting quarantine
> new_quarantine_size to 0 looks fine.
> What would user do with this warning? Number of CPUs and amount of
> memory are generally fixed. Why is it an issue for end user at all? We
> still have some quarantine per-cpu. A WARNING means a [non-critical]
> kernel bug. E.g. syzkaller will catch each and every boot of such
> system as a bug.
How about printk_once then?
Silently setting the quarantine size to zero may puzzle the user.
>
>>> +             new_quarantine_size = 0;
>>> +     } else {
>>> +             new_quarantine_size -= percpu_quarantines;
>>> +     }
>>>       WRITE_ONCE(quarantine_size, new_quarantine_size);
>>>
>>>       last = global_quarantine.head;
>>>



-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-StraÃe, 33
80636 MÃnchen

GeschÃftsfÃhrer: Matthew Scott Sucherman, Paul Terence Manicle
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg