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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
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