linux-kernel - Re: [PATCH] mm/memcg: fix NULL pointer dereference in memcg_slab_free

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Date:   Wed, 28 Jul 2021 18:43:07 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Roman Gushchin <guro@...com>, Wang Hai <wanghai38@...wei.com>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Alexei Starovoitov <ast@...nel.org>,
        wangkefeng.wang@...wei.com, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/memcg: fix NULL pointer dereference in
 memcg_slab_free_hook()

On Wed 28-07-21 07:10:26, Shakeel Butt wrote:
> +Roman
> 
> On Wed, Jul 28, 2021 at 6:23 AM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Wed 28-07-21 17:13:48, Wang Hai wrote:
> > > When I use kfree_rcu() to free a large memory allocated by
> > > kmalloc_node(), the following dump occurs.
> > >
> > > BUG: kernel NULL pointer dereference, address: 0000000000000020
> > > [...]
> > > Oops: 0000 [#1] SMP
> > > [...]
> > > Workqueue: events kfree_rcu_work
> > > RIP: 0010:__obj_to_index include/linux/slub_def.h:182 [inline]
> > > RIP: 0010:obj_to_index include/linux/slub_def.h:191 [inline]
> > > RIP: 0010:memcg_slab_free_hook+0x120/0x260 mm/slab.h:363
> > > [...]
> > > Call Trace:
> > >  kmem_cache_free_bulk+0x58/0x630 mm/slub.c:3293
> > >  kfree_bulk include/linux/slab.h:413 [inline]
> > >  kfree_rcu_work+0x1ab/0x200 kernel/rcu/tree.c:3300
> > >  process_one_work+0x207/0x530 kernel/workqueue.c:2276
> > >  worker_thread+0x320/0x610 kernel/workqueue.c:2422
> > >  kthread+0x13d/0x160 kernel/kthread.c:313
> > >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> > >
> > > When kmalloc_node() a large memory, page is allocated, not slab,
> > > so when freeing memory via kfree_rcu(), this large memory should not
> > > be used by memcg_slab_free_hook(), because memcg_slab_free_hook() is
> > > is used for slab.
> > >
> > > So in this case, there is no need to do anything with this large
> > > page in memcg_slab_free_hook(), just skip it.
> > >
> > > Fixes: 270c6a71460e ("mm: memcontrol/slab: Use helpers to access slab page's memcg_data")
> >
> > Are you sure that this commit is really breaking the code. Unless I have
> > missed something there shouldn't be any real change wrt. large
> > allocations here. page_has_obj_cgroups is just a different name for what
> > what page_objcgs is giving us.
> 
> Actually they are different. For MEMCG_DATA_KMEM page,
> page_has_obj_cgroups() will return false while page_objcgs() on
> non-VM_DEBUG kernels will return "struct obj_cgroup *" instead of
> "struct obj_cgroup **".

Right. Thanks for the clarification. I have missed that subtle
difference.
-- 
Michal Hocko
SUSE Labs