From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f198.google.com (mail-pf1-f198.google.com [209.85.210.198]) by kanga.kvack.org (Postfix) with ESMTP id A79D36B06E2 for ; Fri, 9 Nov 2018 05:53:00 -0500 (EST) Received: by mail-pf1-f198.google.com with SMTP id n22-v6so1209481pff.2 for ; Fri, 09 Nov 2018 02:53:00 -0800 (PST) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id 92-v6sor8367104pli.10.2018.11.09.02.52.59 for (Google Transport Security); Fri, 09 Nov 2018 02:52:59 -0800 (PST) Date: Fri, 9 Nov 2018 21:52:55 +1100 From: Balbir Singh Subject: Re: UBSAN: Undefined behaviour in mm/page_alloc.c Message-ID: <20181109105255.GF9042@350D> References: <20181109084353.GA5321@dhcp22.suse.cz> <20181109095604.GC5321@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181109095604.GC5321@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Tetsuo Handa , Kyungtae Kim , akpm@linux-foundation.org, pavel.tatashin@microsoft.com, vbabka@suse.cz, 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 On Fri, Nov 09, 2018 at 10:56:04AM +0100, 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)); if (unlikely()) might help > > > 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? > -- Balbir Singh.