linux-kernel - Re: [PATCH] Revert "mm/slub: fix a memory leak in sysfs_slab

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Date:   Thu, 28 Jan 2021 13:03:53 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Wang Hai <wanghai38@...wei.com>, torvalds@...ux-foundation.org,
        cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org
Cc:     asmadeus@...ewreck.org, davem@...emloft.net, ericvh@...il.com,
        kuba@...nel.org, lucho@...kov.net, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        v9fs-developer@...ts.sourceforge.net, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        syzbot+d0bd96b4696c1ef67991@...kaller.appspotmail.com
Subject: Re: [PATCH] Revert "mm/slub: fix a memory leak in sysfs_slab_add()"

On 1/28/21 12:32 PM, Wang Hai wrote:
> This reverts commit dde3c6b72a16c2db826f54b2d49bdea26c3534a2.
> 
> syzbot report a double-free bug. The following case can cause this bug.
>  - mm/slab_common.c: create_cache(): if the __kmem_cache_create()
> fails, it does:
> 
> 	out_free_cache:
> 		kmem_cache_free(kmem_cache, s);
> 
>  - but __kmem_cache_create() - at least for slub() - will have done
> 
> 	sysfs_slab_add(s)
> 		-> sysfs_create_group() .. fails ..
> 		-> kobject_del(&s->kobj); .. which frees s ...
> 
> We can't remove the kmem_cache_free() in create_cache(), because
> other error cases of __kmem_cache_create() do not free this.
> 
> So, revert the commit dde3c6b72a16 ("mm/slub: fix a memory leak in
> sysfs_slab_add()") to fix this.
> 
> Reported-by: syzbot+d0bd96b4696c1ef67991@...kaller.appspotmail.com
> Fixes: dde3c6b72a16 ("mm/slub: fix a memory leak in sysfs_slab_add()")
> Signed-off-by: Wang Hai <wanghai38@...wei.com>

Cc: <stable@...r.kernel.org>
Acked-by: Vlastimil Babka <vbabka@...e.cz>

Double-free is worse than a rare small memory leak. Which would still be nice to
fix, but I'm afraid it will be more complicated, so start with preventing the
worse issue, including stable.

> ---
>  mm/slub.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 69742ab9a21d..7ecbbbe5bc0c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5625,10 +5625,8 @@ static int sysfs_slab_add(struct kmem_cache *s)
>  
>  	s->kobj.kset = kset;
>  	err = kobject_init_and_add(&s->kobj, &slab_ktype, NULL, "%s", name);
> -	if (err) {
> -		kobject_put(&s->kobj);
> +	if (err)
>  		goto out;
> -	}
>  
>  	err = sysfs_create_group(&s->kobj, &slab_attr_group);
>  	if (err)
>