linux-kernel - Re: [PATCH v2] mm/memcg: fix NULL pointer dereference in memcg_slab_free

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Date:   Wed, 28 Jul 2021 18:44:03 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Wang Hai <wanghai38@...wei.com>
Cc:     cl@...ux.com, penberg@...nel.org, guro@...com, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org, vbabka@...e.cz,
        hannes@...xchg.org, shakeelb@...gle.com, ast@...nel.org,
        wangkefeng.wang@...wei.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/memcg: fix NULL pointer dereference in
 memcg_slab_free_hook()

On Wed 28-07-21 22:56:55, 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@...wei.com>

This looks like a much better fix. Thanks!

Acked-by: Michal Hocko <mhocko@...e.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;
>  
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs