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Date:   Tue, 20 Nov 2018 00:57:48 +0100
From:   Pavel Machek <pavel@ucw.cz>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Kyungtae Kim <kt0755@gmail.com>, pavel.tatashin@microsoft.com,
        osalvador@suse.de, rppt@linux.vnet.ibm.com, aaron.lu@intel.com,
        iamjoonsoo.kim@lge.com, alexander.h.duyck@linux.intel.com,
        mgorman@techsingularity.net, lifeasageek@gmail.com,
        threeearcat@gmail.com, syzkaller@googlegroups.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c
Message-ID: <20181119235748.GC7367@amd>
References: <CAEAjamseRRHu+TaTkd1TwpLNm8mtDGP=2K0WKLF0wH-3iLcW_w@mail.gmail.com>
 <20181109084353.GA5321@dhcp22.suse.cz>
 <20181113094305.GM15120@dhcp22.suse.cz>
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 <d88fae5c-e12d-ca35-d200-587a2ff02ec9@suse.cz>
 <20181113153204.ea0c0895866838de9e3bc8d0@linux-foundation.org>
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Hi1

> >>>> --- a/mm/page_alloc.c
> >>>> +++ b/mm/page_alloc.c
> >>>> @@ -4364,6 +4353,15 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsign=
ed int order, int preferred_nid,
> >>>>  	gfp_t alloc_mask; /* The gfp_t that was actually used for allocati=
on */
> >>>>  	struct alloc_context ac =3D { };
> >>>> =20
> >>>> +	/*
> >>>> +	 * There are several places where we assume that the order value i=
s sane
> >>>> +	 * so bail out early if the request is out of bound.
> >>>> +	 */
> >>>> +	if (unlikely(order >=3D MAX_ORDER)) {
> >>>> +		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
> >>>> +		return NULL;
> >>>> +	}
> >>>> +
> >>>
> >>> I know "everybody enables CONFIG_DEBUG_VM", but given this is fastpat=
h,
> >>> we could help those who choose not to enable it by using
> >>>
> >>> #ifdef CONFIG_DEBUG_VM
> >>> 	if (WARN_ON_ONCE(order >=3D 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.
> >>
> >=20
> > What does "potential undefined behavior" mean here?
>=20
> I mean that it becomes undefined once a caller with order >=3D 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

--=20
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo=
g.html

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