linux-kernel - Re: [PATCH net-next v3 1/2] nfc: llcp_core: Hold a ref to llcp_local->dev when holding a ref to llcp

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Message-ID: <d41ea6ff-3c29-4a76-833d-19e6a6649d3c@linaro.org>
Date:   Tue, 5 Dec 2023 18:27:28 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Siddh Raman Pant <code@...dh.me>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Suman Ghosh <sumang@...vell.com>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        syzbot+bbe84a4010eeea00982d 
        <syzbot+bbe84a4010eeea00982d@...kaller.appspotmail.com>
Subject: Re: [PATCH net-next v3 1/2] nfc: llcp_core: Hold a ref to
 llcp_local->dev when holding a ref to llcp_local

On 05/12/2023 18:21, Siddh Raman Pant wrote:
> On Tue, 05 Dec 2023 22:10:00 +0530, Krzysztof Kozlowski wrote:
>>> @@ -180,6 +183,7 @@ int nfc_llcp_local_put(struct nfc_llcp_local *local)
>>>  	if (local == NULL)
>>>  		return 0;
>>>  
>>> +	nfc_put_device(local->dev);
>>
>> Mismatched order with get. Unwinding is always in reversed order. Or
>> maybe other order is here on purpose? Then it needs to be explained.
> 
> Yes, local_release() will free local, so local->dev cannot be accessed.
> Will add a comment.

So the problem is just storing the pointer? That's not really the valid
reason.

> 
>>> @@ -959,8 +963,18 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
>>>  	}
>>>  
>>>  	new_sock = nfc_llcp_sock(new_sk);
>>> -	new_sock->dev = local->dev;
>>> +
>>>  	new_sock->local = nfc_llcp_local_get(local);
>>> +	if (!new_sock->local) {
>>
>> There is already an cleanup path/label, so extend it. Existing code
>> needs some improvements in that matter as well.
> 
> Sure.
> 
>>> +		reason = LLCP_DM_REJ;
>>> +		release_sock(&sock->sk);
>>> +		sock_put(&sock->sk);
>>> +		sock_put(&new_sock->sk);
>>> +		nfc_llcp_sock_free(new_sock);
>>> +		goto fail;
>>> +	}
>>> +
>>> +	new_sock->dev = local->dev;
>>>  	new_sock->rw = sock->rw;
>>>  	new_sock->miux = sock->miux;
>>>  	new_sock->nfc_protocol = sock->nfc_protocol;
>>> @@ -1597,7 +1611,13 @@ int nfc_llcp_register_device(struct nfc_dev *ndev)
>>>  	if (local == NULL)
>>>  		return -ENOMEM;
>>>  
>>> -	local->dev = ndev;
>>> +	/* Hold a reference to the device. */
>>
>> That's obvious. Instead write something not obvious - why you call
>> nfc_get_device() while not incrementing reference to llcp_local.
> 
> Should I move it after kref_init()? Here, I'm bailing out early so we
> don't have to do unnecessary init first, and the rest of the function
> will never fail.

I meant, comment is obvious.

> 
>>> +	local->dev = nfc_get_device(ndev->idx);
>>
>> This looks confusing. If you can access ndev->idx, then ndev reference
>> was already increased. In such case iterating through all devices to
>> find it, is unnecessary and confusing.
> 
> I agree, it was something I thought about as well. There should be a
> new function for refcount increment. Maybe the existing one could be
> renamed to nfc_get_device_from_idx() and a new nfc_get_device() be
> defined.
> 
> I didn't want to introduce improvement patches in this UAF series, as
> that would be an independent unit of change.

Best regards,
Krzysztof