[PATCH v1] NFC: reorganize the functions in nci_request

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 <redacted>
Fixes: 6a2968aaf50c ("NFC: basic NCI protocol implementation")
---
 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..1dd0269c1a72 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

Hi there,

Compared to the version-0 patch, this patch add fixes tag in the patch file

+ Fixes: 6a2968aaf50c ("NFC: basic NCI protocol implementation")


Best regards
Lin

On 15/11/2021 15:57, Lin Ma wrote:

Hi there,

Compared to the version-0 patch, this patch add fixes tag in the patch file

+ Fixes: 6a2968aaf50c ("NFC: basic NCI protocol implementation")

Changelog goes either as cover letter (git help format-patch) or after
'---' separator to individual patch.

Best regards,
Krzysztof

Hello:

This patch was applied to netdev/net.git (master)
by Jakub Kicinski [off-list ref]:

On Mon, 15 Nov 2021 22:56:00 +0800 you wrote:

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);    |
                                |

[...]

Here is the summary with links:
  - [v1] NFC: reorganize the functions in nci_request
    https://git.kernel.org/netdev/net/c/86cdf8e38792

You are awesome, thank you!
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