From:   Lin Ma <linma@zju.edu.cn>
To:     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, Lin Ma <linma@zju.edu.cn>
Subject: [PATCH v0] NFC: reorganize the functions in nci_request
Date:   Mon, 15 Nov 2021 22:29:38 +0800
Message-Id: <20211115142938.8109-1-linma@zju.edu.cn>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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().

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;
-- 
2.33.1