linux-kernel - Re: UBSAN: Undefined behaviour in mm/page

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Message-ID: <20181119235748.GC7367@amd>
Date:   Tue, 20 Nov 2018 00:57:48 +0100
From:   Pavel Machek <pavel@....cz>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kyungtae Kim <kt0755@...il.com>, pavel.tatashin@...rosoft.com,
        osalvador@...e.de, rppt@...ux.vnet.ibm.com, aaron.lu@...el.com,
        iamjoonsoo.kim@....com, alexander.h.duyck@...ux.intel.com,
        mgorman@...hsingularity.net, lifeasageek@...il.com,
        threeearcat@...il.com, syzkaller@...glegroups.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c

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
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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