linux-kernel - Re: [PATCH] oom_reaper: close race without using oom

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20170720141138.GJ9058@dhcp22.suse.cz>
Date:   Thu, 20 Jul 2017 16:11:38 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     linux-mm@...ck.org, hannes@...xchg.org, rientjes@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] oom_reaper: close race without using oom_lock

On Wed 19-07-17 05:51:03, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Tue 18-07-17 23:06:50, Tetsuo Handa wrote:
> > > Commit e2fe14564d3316d1 ("oom_reaper: close race with exiting task")
> > > guarded whole OOM reaping operations using oom_lock. But there was no
> > > need to guard whole operations. We needed to guard only setting of
> > > MMF_OOM_REAPED flag because get_page_from_freelist() in
> > > __alloc_pages_may_oom() is called with oom_lock held.
> > > 
> > > If we change to guard only setting of MMF_OOM_SKIP flag, the OOM reaper
> > > can start reaping operations as soon as wake_oom_reaper() is called.
> > > But since setting of MMF_OOM_SKIP flag at __mmput() is not guarded with
> > > oom_lock, guarding only the OOM reaper side is not sufficient.
> > > 
> > > If we change the OOM killer side to ignore MMF_OOM_SKIP flag once,
> > > there is no need to guard setting of MMF_OOM_SKIP flag, and we can
> > > guarantee a chance to call get_page_from_freelist() in
> > > __alloc_pages_may_oom() without depending on oom_lock serialization.
> > > 
> > > This patch makes MMF_OOM_SKIP act as if MMF_OOM_REAPED, and adds a new
> > > flag which acts as if MMF_OOM_SKIP, in order to close both race window
> > > (the OOM reaper side and __mmput() side) without using oom_lock.
> > 
> > Why do we need this patch when
> > http://lkml.kernel.org/r/20170626130346.26314-1-mhocko@kernel.org
> > already removes the lock and solves another problem at once?
> 
> We haven't got an answer from Hugh and/or Andrea whether that patch is safe.

So what? I haven't see anybody disputing the correctness. And to be
honest I really dislike your patch. Yet another round kind of solutions
are just very ugly hacks usually because they are highly timing
sensitive.

> Even if that patch is safe, this patch still helps with CONFIG_MMU=n case.

Could you explain how?
-- 
Michal Hocko
SUSE Labs