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Subject: Re: [PATCH] scsi: zfcp: fix use-after-free in zfcp_unit_remove
To:     Benjamin Block <bblock@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.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>
 <4c65bead-2553-171e-54d2-87a9de0330e8@huawei.com>
 <20201126091353.50cf6ab6.cohuck@redhat.com>
 <20201126094259.GE8578@t480-pf1aa2c2>
From:   Qinglang Miao <miaoqinglang@huawei.com>
Message-ID: <9ba663ad-97fe-6c2a-e15a-45f2de1f0af0@huawei.com>
Date:   Thu, 26 Nov 2020 20:07:32 +0800
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在 2020/11/26 17:42, Benjamin Block 写道:
> On Thu, Nov 26, 2020 at 09:13:53AM +0100, Cornelia Huck wrote:
>> On Thu, 26 Nov 2020 09:27:41 +0800
>> Qinglang Miao <miaoqinglang@huawei.com> wrote:
>>
>>> 在 2020/11/26 1:06, Benjamin Block 写道:
>>>> 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.
>>
>> The issue here (and in the other patches that I had commented on) is
>> that the references have different origins. device_register() acquires
>> a reference, and that reference is given up when you call
>> device_unregister(). However, the code here grabs an extra reference,
>> and it of course has to give it up again when it no longer needs it.
>>
>> This is something that is not that easy to spot by an automated check,
>> I guess?
>>
> 
> Indeed.
> 
> I do think the two patches for zfcp have merit, but not by simply
> removing the put_device(), but by moving it.
> 
> For this patch in particular, I'd think the "proper logic" would be to
> move the `put_device()` to after the `device_unregister()`:
> 
>      device_unregister(&unit->dev);
>      put_device(&unit->dev);
> 
>      return 0;
> 
> As Cornelia pointed out, the extra `get_device()` we do in
> `_zfcp_unit_find()` needs to be reversed, otherwise we have a dangling
> reference and probably some sort of memory-/resource-leak.
> 
> Let's go by example. If we assume the reference count of `unit->dev` is
> R, and the function starts with R = 1 (otherwise the deivce would've
> been freed already), we get:
> 
>      int zfcp_unit_remove(struct zfcp_port *port, u64 fcp_lun)
>      {
>      	struct zfcp_unit *unit;
>      	struct scsi_device *sdev;
>      
>      	write_lock_irq(&port->unit_list_lock);
> // unit->dev (R = 1)
>      	unit = _zfcp_unit_find(port, fcp_lun);
> // get_device(&unit->dev)
> // unit->dev (R = 2)
>      	if (unit)
>      		list_del(&unit->list);
>      	write_unlock_irq(&port->unit_list_lock);
>      
>      	if (!unit)
>      		return -EINVAL;
>      
>      	sdev = zfcp_unit_sdev(unit);
>      	if (sdev) {
>      		scsi_remove_device(sdev);
>      		scsi_device_put(sdev);
>      	}
>      
> // unit->dev (R = 2)
>      	put_device(&unit->dev);
> // unit->dev (R = 1)
>      	device_unregister(&unit->dev);
> // unit->dev (R = 0)
>      
>      	return 0;
>      }
> 
> If we now apply this patch, we'd end up with R = 1 after
> `device_unregister()`, and the device would not be properly removed.
> 
> If you still think that's wrong, then you'll need to better explain why.
> 
Hi Banjamin and Cornelia,

Your replies make me reliaze that I've been holding a mistake 
understanding of put_device() as well as reference count.

Thanks for you two's patient explanation !!

BTW, should I send a v2 on these two patches to move the position of 
put_device()?
>