Subject: Re: [PATCH v0] NFC: reorganize the functions in nci_request
To:     Lin Ma <linma@zju.edu.cn>, netdev@vger.kernel.org
Cc:     krzysztof.kozlowski@canonical.com, davem@davemloft.net,
        kuba@kernel.org, jirislaby@kernel.org, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org
References: <20211115142938.8109-1-linma@zju.edu.cn>
From:   Denis Kirjanov <dkirjanov@suse.de>
Message-ID: <84cf637f-4cda-a7ad-a5d1-484254ef16a5@suse.de>
Date:   Mon, 15 Nov 2021 17:32:35 +0300
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11/15/21 5:29 PM, Lin Ma пишет:
> There is a possible data race as shown below:
> 
> thread-A in nci_request()       | thread-B in nci_close_device()
>                                  | mutex_lock(&ndev->req_lock);
> test_bit(NCI_UP, &ndev->flags); |
> ...                             | test_and_clear_bit(NCI_UP, &ndev->flags)
> mutex_lock(&ndev->req_lock);    |
>                                  |
> 
> This race will allow __nci_request() to be awaked while the device is
> getting removed.
> 
> Similar to commit e2cb6b891ad2 ("bluetooth: eliminate the potential race
> condition when removing the HCI controller"). this patch alters the
> function sequence in nci_request() to prevent the data races between the
> nci_close_device().
Please add Fixes tag
> 
> Signed-off-by: Lin Ma <linma@zju.edu.cn>
> ---
>   net/nfc/nci/core.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/net/nfc/nci/core.c b/net/nfc/nci/core.c
> index 6fd873aa86be..2ab2d6a1a143 100644
> --- a/net/nfc/nci/core.c
> +++ b/net/nfc/nci/core.c
> @@ -144,12 +144,15 @@ inline int nci_request(struct nci_dev *ndev,
>   {
>   	int rc;
>   
> -	if (!test_bit(NCI_UP, &ndev->flags))
> -		return -ENETDOWN;
> -
>   	/* Serialize all requests */
>   	mutex_lock(&ndev->req_lock);
> -	rc = __nci_request(ndev, req, opt, timeout);
> +	/* check the state after obtaing the lock against any races
> +	 * from nci_close_device when the device gets removed.
> +	 */
> +	if (test_bit(NCI_UP, &ndev->flags))
> +		rc = __nci_request(ndev, req, opt, timeout);
> +	else
> +		rc = -ENETDOWN;
>   	mutex_unlock(&ndev->req_lock);
>   
>   	return rc;
>