Re: UBSAN: Undefined behaviour in mm/page_alloc.c

[Vlastimil Babka]

On 11/9/18 10:56 AM, Michal Hocko wrote:
> On Fri 09-11-18 18:41:53, Tetsuo Handa wrote:
>> On 2018/11/09 17:43, Michal Hocko wrote:
>>> @@ -4364,6 +4353,17 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
>>>  	gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
>>>  	struct alloc_context ac = { };
>>>  
>>> +	/*
>>> +	 * In the slowpath, we sanity check order to avoid ever trying to
>>
>> Please keep the comment up to dated.
> 
> Does this following look better?
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 9fc10a1029cf..bf9aecba4222 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4354,10 +4354,8 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
>  	struct alloc_context ac = { };
>  
>  	/*
> -	 * In the slowpath, we sanity check order to avoid ever trying to
> -	 * reclaim >= MAX_ORDER areas which will never succeed. Callers may
> -	 * be using allocators in order of preference for an area that is
> -	 * too large.
> +	 * There are several places where we assume that the order value is sane
> +	 * so bail out early if the request is out of bound.
>  	 */
>  	if (order >= MAX_ORDER) {
>  		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));

Looks ok, but I'd add unlikely(), although it doesn't currently seem to
make any difference.

You can add Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

>> I don't like that comments in OOM code is outdated.
>>
>>> +	 * reclaim >= MAX_ORDER areas which will never succeed. Callers may
>>> +	 * be using allocators in order of preference for an area that is
>>> +	 * too large.
>>> +	 */
>>> +	if (order >= MAX_ORDER) {
>>
>> Also, why not to add BUG_ON(gfp_mask & __GFP_NOFAIL); here?
> 
> Because we do not want to blow up the kernel just because of a stupid
> usage of the allocator. Can you think of an example where it would
> actually make any sense?
> 
> I would argue that such a theoretical abuse would blow up on an
> unchecked NULL ptr access. Isn't that enough?

Agreed.