Re: [PATCH v2 3/3] nbd: fix race between nbd_alloc_config() and module removal

[Hou Tao <houtao1@huawei.com>]

Hi Christoph,

Any comments for this patch ?

On 9/7/2021 11:04 AM, Hou Tao wrote:
> Hi,
>
> On 9/6/2021 6:25 PM, Christoph Hellwig wrote:
>> On Mon, Sep 06, 2021 at 06:08:54PM +0800, Hou Tao wrote:
>>>>> +	if (!try_module_get(THIS_MODULE))
>>>>> +		return ERR_PTR(-ENODEV);
>>>> try_module_get(THIS_MODULE) is an indicator for an unsafe pattern.  If
>>>> we don't already have a reference it could never close the race.
>>>>
>>>> Looking at the callers:
>>>>
>>>>  - nbd_open like all block device operations must have a reference
>>>>    already.
>>> Yes. nbd_open() has already taken a reference in dentry_open().
>>>>  - for nbd_genl_connect I'm not an expert, but given that struct
>>>>    nbd_genl_family has a module member I suspect the networkinh
>>>>    code already takes a reference.
>>> That was my original though, but the fact is netlink code doesn't take a module reference
>>>
>>> in genl_family_rcv_msg_doit() and netlink uses genl_lock_all() to serialize between module removal
>>>
>>> and nbd_connect_genl_ops calling, so I think use try_module_get() is OK here.
>> How it this going to work?  If there was a race you just shortened it,
>> but it can still happen before you call try_module_get.  So I think we
>> need to look into how the netlink calling conventions are supposed to
>> look and understand the issues there first.
>> .
> Let me explain first. The reason it works is due to genl_lock_all() in netlink code.
>
> If the module removal happens before calling try_module_get(), nbd_cleanup() will
>
> call genl_unregister_family() first, and then genl_lock_all(). genl_lock_all() will
>
> prevent ops in nbd_connect_genl_ops() from being called, because the calling
>
> of nbd ops happens in genl_rcv() which needs to acquire cb_lock first.
>
>
> process A                                       process B
>
> module removal
>
> genl_unregister_family()
>
>   genl_lock_all()
>
>     down_write(&cb_lock)
>
>                                                 receive a new netlink message
>
>                                                 genl_rcv()
>
>                                                    // will wait for the removal of nbd ops
>
>                                                    down_read(&cb_lock)
>
> If nbd_alloc_config() happens before the module removal, genl_rcv() must
>
> have been acquired cb_lock & genl_mutex, so nbd_cleanup() will block in
>
> genl_unregister_family(). When nbd_alloc_config() calls try_module_get(),
>
> it will find out the module is dying, so fail nbd_genl_connect().
>
>
> process A                                     process B
>
> a new netlink message
>
> genl_rcv()
>
>   down_read(&cb_lock)
>
>     mutex_lock(&genl_mutex)
>
>       nbd_genl_connect()
>
>         nbd_alloc_config()
>
>                                                module removal
>
>                                                genl_unregister_family
>
>           // module is dying, so fail
>
>           try_module_get()
>
>                                                  genl_lock_all()
>
>                                                    // wait for the completion of nbd ops
>
>                                                    down_write(&cb_lock)
>
> I have checked multiple genl_ops, it seems that the reason why these genl_ops
>
> don't need try_module_get() is that these ops don't create new object through
>
> genl_ops and just control it. However genl_family_rcv_msg_dumpit() will try to
>
> call try_module_get(), but according to the history (6dc878a8ca39 "netlink: add reference of module in netlink_dump_start"),
>
> it is because inet_diag_handler_cmd() will call __netlink_dump_start().
>
> Regards,
>
> Tao
>
>
> .