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Subject: Re: [PATCH v2] mm/memcg: fix NULL pointer dereference in
 memcg_slab_free_hook()
To:     Wang Hai <wanghai38@huawei.com>, <cl@linux.com>,
        <penberg@kernel.org>, <guro@fb.com>, <rientjes@google.com>,
        <iamjoonsoo.kim@lge.com>, <akpm@linux-foundation.org>,
        <vbabka@suse.cz>, <hannes@cmpxchg.org>, <shakeelb@google.com>,
        <ast@kernel.org>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
References: <20210728145655.274476-1-wanghai38@huawei.com>
From:   Kefeng Wang <wangkefeng.wang@huawei.com>
Message-ID: <277efd2a-e2eb-5d45-150b-8885b571eef7@huawei.com>
Date:   Thu, 29 Jul 2021 14:48:44 +0800
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On 2021/7/28 22:56, 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.
>
> Using page_objcgs_check() instead of page_objcgs() in
> memcg_slab_free_hook() to fix this bug.
>
> Fixes: 270c6a71460e ("mm: memcontrol/slab: Use helpers to access slab page's memcg_data")
> Signed-off-by: Wang Hai <wanghai38@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> v1->v2: Use page_objcgs_check() to fix this bug
>   mm/slab.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 67e06637ff2e..59db4797acd4 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -339,7 +339,7 @@ static inline void memcg_slab_free_hook(struct kmem_cache *s_orig,
>   			continue;
>   
>   		page = virt_to_head_page(p[i]);
> -		objcgs = page_objcgs(page);
> +		objcgs = page_objcgs_check(page);
>   		if (!objcgs)
>   			continue;
>