linux-kernel - Re: [PATCH -mm 2/2] memcg: fix css reference leak and endless loop in mem_cgroup

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: <20140122081212.GA18154@dhcp22.suse.cz>
Date:	Wed, 22 Jan 2014 09:12:12 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Greg Thelen <gthelen@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 2/2] memcg: fix css reference leak and endless loop
 in mem_cgroup_iter

On Tue 21-01-14 11:42:19, Andrew Morton wrote:
> On Tue, 21 Jan 2014 11:45:43 +0100 Michal Hocko <mhocko@...e.cz> wrote:
> 
> > 19f39402864e (memcg: simplify mem_cgroup_iter) has reorganized
> > mem_cgroup_iter code in order to simplify it. A part of that change was
> > dropping an optimization which didn't call css_tryget on the root of
> > the walked tree. The patch however didn't change the css_put part in
> > mem_cgroup_iter which excludes root.
> > This wasn't an issue at the time because __mem_cgroup_iter_next bailed
> > out for root early without taking a reference as cgroup iterators
> > (css_next_descendant_pre) didn't visit root themselves.
> > 
> > Nevertheless cgroup iterators have been reworked to visit root by
> > bd8815a6d802 (cgroup: make css_for_each_descendant() and friends include
> > the origin css in the iteration) when the root bypass have been dropped
> > in __mem_cgroup_iter_next. This means that css_put is not called for
> > root and so css along with mem_cgroup and other cgroup internal object
> > tied by css lifetime are never freed.
> > 
> > Fix the issue by reintroducing root check in __mem_cgroup_iter_next
> > and do not take css reference for it.
> > 
> > This reference counting magic protects us also from another issue, an
> > endless loop reported by Hugh Dickins when reclaim races with root
> > removal and css_tryget called by iterator internally would fail. There
> > would be no other nodes to visit so __mem_cgroup_iter_next would return
> > NULL and mem_cgroup_iter would interpret it as "start looping from root
> > again" and so mem_cgroup_iter would loop forever internally.
> 
> I grabbed these two patches but I will sit on them for a week or so,
> pending review-n-test.

Yes, there is no rush and this needs a proper review.

> > Cc: stable@...r.kernel.org # mem_leak part 3.12+
> 
> What does this mean?

Dohh. I had both patches in one but then I decided to split it. This is
just left over. Should be 3.12+.

Sorry about the confusion.

-- 
Michal Hocko
SUSE Labs
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/