Re: [PATCH] include/linux/kasan.h: Notice about 0 for kasan_[dis/en]able_current()

[Chen Gang]

On 5/2/16 19:23, Alexander Potapenko wrote:
> On Mon, May 2, 2016 at 1:20 PM, Chen Gang <chengang@xxxxxxxxxxxxxxxx> wrote:
>> On 5/2/16 18:49, Alexander Potapenko wrote:
>>> On Mon, May 2, 2016 at 7:35 AM,  <chengang@xxxxxxxxxxxxxxxx> wrote:
>>>>
>>>> According to their comments and the kasan_depth's initialization, if
>>>> kasan_depth is zero, it means disable. So kasan_depth need consider
>>>> about the 0 overflow.
>>>>
>>>> Also remove useless comments for dummy kasan_slab_free().
>>>>
>>>> Signed-off-by: Chen Gang <gang.chen.5i5j@xxxxxxxxx>
>>>
>>> Acked-by: Alexander Potapenko <glider@xxxxxxxxxx>
> Nacked-by: Alexander Potapenko <glider@xxxxxxxxxx>
>>>
>>
>> OK, thanks.
> Well, on a second thought I take that back, there still might be problems.
> I haven't noticed the other CL, and was too hasty reviewing this one.
> 
> As kasan_disable_current() and kasan_enable_current() always go
> together, we need to prevent nested calls to them from breaking
> everything.
> If we ignore some calls to kasan_disable_current() to prevent
> overflows, the pairing calls to kasan_enable_current() will bring
> |current->kasan_depth| to an invalid state.
> 
> E.g. if I'm understanding your idea correctly, after the following
> sequence of calls:
>   kasan_disable_current();  // #1
>   kasan_disable_current();  // #2
>   kasan_enable_current();  // #3
>   kasan_enable_current();  // #4
> 
> the value of |current->kasan_depth| will be 2, so a single subsequent
> call to kasan_disable_current() won't disable KASAN.
> 
> I think we'd better add BUG checks to bail out if the value of
> |current->kasan_depth| is too big or too small.
> 

For me, BUG_ON is OK. e.g.

 - BUG_ON(!kasan_depth) as soon as be in kasan_enable_current().

 - BUG_ON(!(kasan_depth - 1)) as soon as be in kasan_disable_current().

Welcome another members ideas, if no any additional reply within 3 days,
I shall send patch v2 for it.


Thanks.
-- 
Chen Gang (éå)

Managing Natural Environments is the Duty of Human Beings.