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Subject: Re: kobject_init_and_add is easy to misuse
To:     Matthew Wilcox <willy@infradead.org>,
        Wang Hai <wanghai38@huawei.com>
Cc:     cl@linux.com, penberg@kernel.org, rientjes@google.com,
        iamjoonsoo.kim@lge.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
References: <20200602115033.1054-1-wanghai38@huawei.com>
 <20200602121035.GL19604@bombadil.infradead.org>
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Message-ID: <d7c1a000-ca47-bace-c098-d3f3ab0c5127@yandex-team.ru>
Date:   Tue, 2 Jun 2020 16:48:48 +0300
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On 02/06/2020 15.10, Matthew Wilcox wrote:
> On Tue, Jun 02, 2020 at 07:50:33PM +0800, Wang Hai wrote:
>> syzkaller reports for memory leak when kobject_init_and_add()
>> returns an error in the function sysfs_slab_add() [1]
>>
>> When this happened, the function kobject_put() is not called for the
>> corresponding kobject, which potentially leads to memory leak.
>>
>> This patch fixes the issue by calling kobject_put() even if
>> kobject_init_and_add() fails.
> 
> I think this speaks to a deeper problem with kobject_init_and_add()
> -- the need to call kobject_put() if it fails is not readily apparent
> to most users.  This same bug appears in the first three users of
> kobject_init_and_add() that I checked --
> arch/ia64/kernel/topology.c
> drivers/firmware/dmi-sysfs.c
> drivers/firmware/efi/esrt.c
> drivers/scsi/iscsi_boot_sysfs.c
> 
> Some do get it right --
> arch/powerpc/kernel/cacheinfo.c
> drivers/gpu/drm/ttm/ttm_bo.c
> drivers/gpu/drm/ttm/ttm_memory.c
> drivers/infiniband/hw/mlx4/sysfs.c
> 
> I'd argue that the current behaviour is wrong, that kobject_init_and_add()
> should call kobject_put() if the add fails.  This would need a tree-wide
> audit.  But somebody needs to do that anyway because based on my random
> sampling, half of the users currently get it wrong.
> 

At his point kobject doesn't own kmem-cache structure itself yet.

So calling kobject_put() will free kmem-cache and then it will be
freed second time on error path in create_cache().

I suppose freeing in case of error should be pushed from common
create_cache() into slab-specific __kmem_cache_create().