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Subject: Re: [PATCH] nbd: pass nbd_sock to nbd_read_reply() instead of index
To:     Ming Lei <ming.lei@redhat.com>, Yu Kuai <yukuai1@huaweicloud.com>
Cc:     linan666@huaweicloud.com, josef@toxicpanda.com, axboe@kernel.dk,
        linux-block@vger.kernel.org, nbd@other.debian.org,
        linux-kernel@vger.kernel.org, linan122@huawei.com,
        yi.zhang@huawei.com, houtao1@huawei.com, yangerkun@huawei.com,
        "yukuai (C)" <yukuai3@huawei.com>
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From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <bbadaad4-172e-af7b-2a47-52f7e7c83423@huaweicloud.com>
Date:   Thu, 28 Sep 2023 17:40:42 +0800
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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@huaweicloud.com wrote:
>>>>>>>> From: Li Nan <linan122@huawei.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.

Kuai

> 
> 
> Thanks,
> Ming
> 
> .
>