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Subject: Re: [PATCH RFC 1/2] kobject: add return value for kobject_put()
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     hch@lst.de, axboe@kernel.dk, willy@infradead.org,
        martin.petersen@oracle.com, kch@nvidia.com,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        yukuai1@huaweicloud.com, yi.zhang@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20221018131432.434167-1-yukuai3@huawei.com>
 <20221018131432.434167-2-yukuai3@huawei.com> <Y06je6LiDicUfzto@kroah.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <2f962069-8fd9-08df-aa00-062b94569c36@huaweicloud.com>
Date:   Tue, 18 Oct 2022 21:12:08 +0800
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?? 2022/10/18 21:00, Greg KH д??:
> On Tue, Oct 18, 2022 at 09:14:31PM +0800, Yu Kuai wrote:
>> The return value will be used in later patch to fix uaf for slave_dir
>> and bd_holder_dir in block layer.
> 
> Then the user will be incorrect, this is not ok, you should never care
> if you are the last "put" on an object at all.  Hint, what happens right
> after you call this and get the result?
> 

I tried to reset the pointer to NULL in patch 2 to prevent uaf. And the
whole kobject_put() and pointer reset is protected by a mutex, the mutex
will be used on the reader side before kobject_get as well. So, in fact,
I'm protecting them by the mutex...

I can bypass it by using another reference anyway. But let's see if
anyone has suggestions on the other patch.

> sorry, but NAK.

I know the best way is too refactor the lifecycle of the problematic
bd_holder_dir/slave_dir, however, I gave that up because this seems
quite complicated and influence is very huge...

Thanks,
Kuai
> 
> greg k-h
> .
>