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