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

[Pavel Machek]

Hi1

> >>>> --- a/mm/page_alloc.c
> >>>> +++ b/mm/page_alloc.c
> >>>> @@ -4364,6 +4353,15 @@ __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 = { };
> >>>>  
> >>>> +	/*
> >>>> +	 * 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 (unlikely(order >= MAX_ORDER)) {
> >>>> +		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
> >>>> +		return NULL;
> >>>> +	}
> >>>> +
> >>>
> >>> I know "everybody enables CONFIG_DEBUG_VM", but given this is fastpath,
> >>> we could help those who choose not to enable it by using
> >>>
> >>> #ifdef CONFIG_DEBUG_VM
> >>> 	if (WARN_ON_ONCE(order >= MAX_ORDER && !(gfp_mask & __GFP_NOWARN)))
> >>> 		return NULL;
> >>> #endif
> >>
> >> Hmm, but that would mean there's still potential undefined behavior for
> >> !CONFIG_DEBUG_VM, so I would prefer not to do it like that.
> >>
> > 
> > What does "potential undefined behavior" mean here?
> 
> I mean that it becomes undefined once a caller with order >= MAX_ORDER
> appears. Worse if it's directly due to a userspace action, like in this
> case.

We should really check if value from userspace is sane _before_
passing it to alloc_pages(). Anything else is too fragile. Maybe
alloc_pages should do the second check, but...

									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
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