linux-kernel - Re: [PATCH] nbd: pass nbd_sock to nbd_read

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Date:   Mon, 30 Oct 2023 10:07:13 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Yu Kuai <yukuai1@...weicloud.com>, Ming Lei <ming.lei@...hat.com>
Cc:     linan666@...weicloud.com, josef@...icpanda.com, axboe@...nel.dk,
        linux-block@...r.kernel.org, nbd@...er.debian.org,
        linux-kernel@...r.kernel.org, linan122@...wei.com,
        yi.zhang@...wei.com, houtao1@...wei.com, yangerkun@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] nbd: pass nbd_sock to nbd_read_reply() instead of index

Hi,

在 2023/09/28 17:40, Yu Kuai 写道:
> Hi,
> 
> 在 2023/09/28 17:24, Ming Lei 写道:
>> On Thu, Sep 28, 2023 at 05:06:40PM +0800, Yu Kuai wrote:
>>> Hi,
>>>
>>> 在 2023/09/28 16:57, Ming Lei 写道:
>>>> On Thu, Sep 28, 2023 at 04:55:03PM +0800, Yu Kuai wrote:
>>>>> Hi,
>>>>>
>>>>> 在 2023/09/28 15:40, Ming Lei 写道:
>>>>>> On Thu, Sep 28, 2023 at 02:03:28PM +0800, Yu Kuai wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> 在 2023/09/28 12:05, Ming Lei 写道:
>>>>>>>> On Mon, Sep 11, 2023 at 10:33:08AM +0800, 
>>>>>>>> linan666@...weicloud.com wrote:
>>>>>>>>> From: Li Nan <linan122@...wei.com>
>>>>>>>>>
>>>>>>>>> If a socket is processing ioctl 'NBD_SET_SOCK', config->socks 
>>>>>>>>> might be
>>>>>>>>> krealloc in nbd_add_socket(), and a garbage request is received 
>>>>>>>>> now, a UAF
>>>>>>>>> may occurs.
>>>>>>>>>
>>>>>>>>>       T1
>>>>>>>>>       nbd_ioctl
>>>>>>>>>        __nbd_ioctl
>>>>>>>>>         nbd_add_socket
>>>>>>>>>          blk_mq_freeze_queue
>>>>>>>>>                 T2
>>>>>>>>>                       recv_work
>>>>>>>>>                        nbd_read_reply
>>>>>>>>>                         sock_xmit
>>>>>>>>>          krealloc config->socks
>>>>>>>>>                    def config->socks
>>>>>>>>>
>>>>>>>>> Pass nbd_sock to nbd_read_reply(). And introduce a new function
>>>>>>>>> sock_xmit_recv(), which differs from sock_xmit only in the way 
>>>>>>>>> it get
>>>>>>>>> socket.
>>>>>>>>>
>>>>>>>>
>>>>>>>> I am wondering why not grab queue usage counter before calling 
>>>>>>>> nbd_read_reply()
>>>>>>>> for avoiding such issue, something like the following change:
>>>>>>>>
>>>>>>>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>>>>>>>> index df1cd0f718b8..09215b605b12 100644
>>>>>>>> --- a/drivers/block/nbd.c
>>>>>>>> +++ b/drivers/block/nbd.c
>>>>>>>> @@ -837,9 +837,6 @@ static void recv_work(struct work_struct *work)
>>>>>>>>          while (1) {
>>>>>>>>              struct nbd_reply reply;
>>>>>>>> -        if (nbd_read_reply(nbd, args->index, &reply))
>>>>>>>> -            break;
>>>>>>>> -
>>>>>>>>              /*
>>>>>>>>               * Grab .q_usage_counter so request pool won't go 
>>>>>>>> away, then no
>>>>>>>>               * request use-after-free is possible during 
>>>>>>>> nbd_handle_reply().
>>>>>>>> @@ -852,6 +849,9 @@ static void recv_work(struct work_struct *work)
>>>>>>>>                  break;
>>>>>>>>              }
>>>>>>>
>>>>>>> This break how nbd works, if there is no reply yet, recv_work() will
>>>>>>> wait for reply in:
>>>>>>>
>>>>>>> nbd_read_reply
>>>>>>>     sock_xmit
>>>>>>>      sock_recvmsg
>>>>>>>
>>>>>>> After this change, recv_work() will just return if there is no io.
>>>>>>
>>>>>> OK, got it, thanks for the input.
>>>>>>
>>>>>> But I feel it isn't necessary & fragile to store one extra 
>>>>>> reference of nsock in
>>>>>> `recv_thread_args`.
>>>>>>
>>>>>> Just run a quick look, the only potential UAF on config->socks 
>>>>>> should be recv_work(),
>>>>>> so you can retrieve the `nsock` reference at the entry of 
>>>>>> recv_work(),
>>>>>
>>>>> I don't understand what you mean retrieve the 'nsock', is following 
>>>>> what
>>>>> you expected?
>>>>>
>>>>> blk_queue_enter() -> prevent concurrent with nbd_add_socket
>>>>> nsock = config->socks[args->index]
>>>>> blk_queue_exit()
>>>>
>>>> Yeah, turns out you do understand, :-)
>>>
>>> Ok, I was not sure about this blk_queue_enter(). By the way, this
>>
>> blk_queue_enter() isn't exported, but you can grab ->config_lock
>> for getting the `nsock`.
>>
>>> remind me of what you did to fix uaf of access queue->mq_hctx[] by
>>> convert the array to xarray.
>>>
>>>
>>> Maybe it's better to covert config->socks[] to xarray to fix this uaf as
>>> well?
>>
>> ->socks[idx] is needed in nbd fast path, so xarray may not be one good 
>> idea
>> since xarray_load() introduces extra load, especially ->socks[] uaf
>> should exist in recv_work() very likely. For other cases, the active
>> block request holds queue usage counter.
> 
> Thanks for the explanation, grab 'config_lock' to get 'nsock' in the
> begining sounds good to me.

After reviewing some code, I found that it's wrong to grab config_lock,
because other context will grab such lock and flush_workqueue(), and
there is no gurantee that recv_work() will grab the lock first.

Will it be acceptable to export blk_queue_enter()? I can't think of
other way to retrieve the`nsock` reference at the entry of recv_work().

Thanks,
Kuai

> 
> Kuai
> 
>>
>>
>> Thanks,
>> Ming
>>
>> .
>>
> 
> .
>