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Subject: Re: [PATCH] scsi: zfcp: fix use-after-free in zfcp_unit_remove
To:     Benjamin Block <bblock@linux.ibm.com>
CC:     Steffen Maier <maier@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        <linux-s390@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <linux-scsi@vger.kernel.org>
References: <20201120074854.31754-1-miaoqinglang@huawei.com>
 <20201125170658.GB8578@t480-pf1aa2c2>
From:   Qinglang Miao <miaoqinglang@huawei.com>
Message-ID: <4c65bead-2553-171e-54d2-87a9de0330e8@huawei.com>
Date:   Thu, 26 Nov 2020 09:27:41 +0800
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?? 2020/11/26 1:06, Benjamin Block д??:
> On Fri, Nov 20, 2020 at 03:48:54PM +0800, Qinglang Miao wrote:
>> kfree(port) is called in put_device(&port->dev) so that following
>> use would cause use-after-free bug.
>>
>> The former put_device is redundant for device_unregister contains
>> put_device already. So just remove it to fix this.
>>
>> Fixes: 86bdf218a717 ("[SCSI] zfcp: cleanup unit sysfs attribute usage")
>> Reported-by: Hulk Robot <hulkci@huawei.com>
>> Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
>> ---
>>   drivers/s390/scsi/zfcp_unit.c | 2 --
>>   1 file changed, 2 deletions(-)
>>
>> diff --git a/drivers/s390/scsi/zfcp_unit.c b/drivers/s390/scsi/zfcp_unit.c
>> index e67bf7388..664b77853 100644
>> --- a/drivers/s390/scsi/zfcp_unit.c
>> +++ b/drivers/s390/scsi/zfcp_unit.c
>> @@ -255,8 +255,6 @@ int zfcp_unit_remove(struct zfcp_port *port, u64 fcp_lun)
>>   		scsi_device_put(sdev);
>>   	}
>>   
>> -	put_device(&unit->dev);
>> -
>>   	device_unregister(&unit->dev);
>>  >>   	return 0;
> 
> Same as in the other mail for `zfcp_sysfs_port_remove_store()`. We
> explicitly get a new ref in `_zfcp_unit_find()`, so we also need to put
> that away again.
>
Sorry, Benjamin, I don't think so, because device_unregister calls 
put_device inside.

It seem's that another put_device before or after device_unregister is 
useless and even might cause an use-after-free.