linux-kernel - Re: [PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

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Date:   Sun, 9 Feb 2020 21:16:47 -0600
From:   Mike Christie <mchristi@...hat.com>
To:     "sunke (E)" <sunke32@...wei.com>, josef@...icpanda.com,
        axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, nbd@...er.debian.org,
        linux-kernel@...r.kernel.org, Xiubo Li <xiubli@...hat.com>
Subject: Re: [PATCH] nbd: fix potential NULL pointer fault in connect and
 disconnect process

On 01/19/2020 01:10 AM, sunke (E) wrote:
> 
> Thanks for your detailed suggestions.
> 
> 在 2020/1/18 1:32, Mike Christie 写道:
>> On 01/17/2020 05:50 AM, Sun Ke wrote:
>>> Connect and disconnect a nbd device repeatedly, will cause
>>> NULL pointer fault.
>>>
>>> It will appear by the steps:
>>> 1. Connect the nbd device and disconnect it, but now nbd device
>>>     is not disconnected totally.
>>> 2. Connect the same nbd device again immediately, it will fail
>>>     in nbd_start_device with a EBUSY return value.
>>> 3. Wait a second to make sure the last config_refs is reduced
>>>     and run nbd_config_put to disconnect the nbd device totally.
>>> 4. Start another process to open the nbd_device, config_refs
>>>     will increase and at the same time disconnect it.
>>
>> Just to make sure I understood this, for step 4 the process is doing:
>>
>> open(/dev/nbdX);
>> ioctl(NBD_DISCONNECT, /dev/nbdX) or nbd_genl_disconnect(for /dev/nbdX)
>>
>> ?
>>
> do nbd_genl_disconnect(for /dev/nbdX)；
> I tested it. Connect /dev/nbdX
> through ioctl interface by nbd-client -L -N export localhost /dev/nbdX and
> through netlink interface by nbd-client localhost XXXX /dev/nbdX,
> disconnect /dev/nbdX by nbd-client -d /dev/nbdX.
> Both call nbd_genl_disconnect(for /dev/nbdX) and both contain the same
> null pointer dereference.
>> There is no successful NBD_DO_IT / nbd_genl_connect between the open and
>> disconnect calls at step #4, because it would normally be done at #2 and
>> that failed. nbd_disconnect_and_put could then reference a null
>> recv_workq. If we are also racing with a close() then that could free
>> the device/config from under nbd_disconnect_and_put.
>>
> Yes, nbd_disconnect_and_put could then reference a null recv_workq.

Hey Sunke

How about the attached patch. I am still testing it. The basic idea is
that we need to do a flush whenever we have done a sock_shutdown and are
in the disconnect/connect/clear sock path, so it just adds the flush in
that function. We then do not need to keep adding these flushes everywhere.

View attachment "0001-nbd-fix-crash-during-nbd_genl_disconnect.patch" of type "text/x-patch" (5018 bytes)