Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 9 Nov 2018 09:56:09 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Kyungtae Kim <kt0755@gmail.com>, 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,
        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: <20181109095609.GC23260@techsingularity.net>
References: <CAEAjamseRRHu+TaTkd1TwpLNm8mtDGP=2K0WKLF0wH-3iLcW_w@mail.gmail.com>
 <20181109084353.GA5321@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-15
Content-Disposition: inline
In-Reply-To: <20181109084353.GA5321@dhcp22.suse.cz>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Nov 09, 2018 at 09:43:53AM +0100, Michal Hocko wrote:
> On Thu 08-11-18 23:09:23, Kyungtae Kim wrote:
> > We report a bug in v4.19-rc2 (4.20-rc1 as well, I guess):
> > 
> > kernel config: https://kt0755.github.io/etc/config_v2-4.19
> > repro: https://kt0755.github.io/etc/repro.c4074.c
> > 
> > In the middle of page request, this arose because order is too large to handle
> >  (mm/page_alloc.c:3119). It actually comes from that order is
> > controllable by user input
> > via raw_cmd_ioctl without its sanity check, thereby causing memory problem.
> > To stop it, we can use like MAX_ORDER for bounds check before using it.
> 
> Yes, we do only check the max order in the slow path. We have already
> discussed something similar with Konstantin [1][2]. Basically kvmalloc
> for a large size might get to the page allocator with an out of bound
> order and warn during direct reclaim.
> 
> I am wondering whether really want to check for the order in the fast
> path instead. I have hard time to imagine this could cause a measurable
> impact.
> 
> The full patch is below
> 
> [1] http://lkml.kernel.org/r/154109387197.925352.10499549042420271600.stgit@buzz
> [2] http://lkml.kernel.org/r/154106356066.887821.4649178319705436373.stgit@buzz
> 

I'm ok with such changes under the policy "there is no point being fast if
we're broken". It's unfortunate and I know the original microoptimisation
was mine but if the fast-path check ends up being a problem then I/we go
back to finding ways of making the page allocator faster from a fundamental
algorithmic point of view and not a microoptimisation approach. There is
potential fruit there, just none that is low-hanging.

-- 
Mel Gorman
SUSE Labs