[PATCH nfc] nfc: nci: fix double completion race in nci_data_exchange_complete

[Zhenghang Xiao <kipreyyy@gmail.com>]

nci_close_device() and nci_rx_work can both call
nci_data_exchange_complete() concurrently.  After commit 4527025d440ce8
("nfc: nci: fix circular locking dependency in nci_close_device") moved
flush_workqueue(ndev->rx_wq) after mutex_unlock(&ndev->req_lock),
rx_work is no longer serialized with the explicit completion call in the
close path.  Both callers read the non-NULL callback pointer and invoke
rawsock_data_exchange_complete(), which calls sock_put() -- but only one
sock_hold() was taken, so the second sock_put() underflows the refcount
and frees the socket while it is still in use.

Replace the bare clear_bit(NCI_DATA_EXCHANGE) with
test_and_clear_bit() so that only the first caller proceeds to invoke
the callback.

Fixes: 4527025d440c ("nfc: nci: fix circular locking dependency in nci_close_device")
Signed-off-by: Zhenghang Xiao <kipreyyy@gmail.com>
---
 net/nfc/nci/data.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/net/nfc/nci/data.c b/net/nfc/nci/data.c
index 5f98c73db5af..4253edea5c8d 100644
--- a/net/nfc/nci/data.c
+++ b/net/nfc/nci/data.c
@@ -46,11 +46,11 @@ void nci_data_exchange_complete(struct nci_dev *ndev, struct sk_buff *skb,
 	timer_delete_sync(&ndev->data_timer);
 	clear_bit(NCI_DATA_EXCHANGE_TO, &ndev->flags);
 
-	/* Mark the exchange as done before calling the callback.
-	 * The callback (e.g. rawsock_data_exchange_complete) may
-	 * want to immediately queue another data exchange.
-	 */
-	clear_bit(NCI_DATA_EXCHANGE, &ndev->flags);
+	/* Claim completion atomically -- both close and rx_work may race here */
+	if (!test_and_clear_bit(NCI_DATA_EXCHANGE, &ndev->flags)) {
+		kfree_skb(skb);
+		return;
+	}
 
 	if (cb) {
 		/* forward skb to nfc core */
-- 
2.50.1 (Apple Git-155)