linux-kernel - Re: [PATCH v2] mm/slub: fix a deadlock in show_slab

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: <20191007081621.GE2381@dhcp22.suse.cz>
Date:   Mon, 7 Oct 2019 10:16:21 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, tj@...nel.org, vdavydov.dev@...il.com,
        hannes@...xchg.org, guro@...com, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] mm/slub: fix a deadlock in show_slab_objects()

On Fri 04-10-19 14:57:01, Michal Hocko wrote:
> On Fri 04-10-19 08:31:49, Qian Cai wrote:
> > Long time ago, there fixed a similar deadlock in show_slab_objects()
> > [1]. However, it is apparently due to the commits like 01fb58bcba63
> > ("slab: remove synchronous synchronize_sched() from memcg cache
> > deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
> > kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
> > just reading files in /sys/kernel/slab which will generate a lockdep
> > splat below.
> > 
> > Since the "mem_hotplug_lock" here is only to obtain a stable online node
> > mask while racing with NUMA node hotplug, in the worst case, the results
> > may me miscalculated while doing NUMA node hotplug, but they shall be
> > corrected by later reads of the same files.
> 
> I think it is important to mention that this doesn't expose the
> show_slab_objects to use-after-free. There is only a single path that
> might really race here and that is the slab hotplug notifier callback
> __kmem_cache_shrink (via slab_mem_going_offline_callback) but that path
> doesn't really destroy kmem_cache_node data structures.

Andrew, please add this to the changelog so that we do not have to
scratch heads again when looking into that code.

Thanks!
-- 
Michal Hocko
SUSE Labs