NFC: digital: Clear poll_tech_count before activating target

Currently, digital_target_found() has a race between
the events started by calling nfc_targets_found()
(which ultimately expect ddev->poll_tech_count to be
zero) and setting ddev->poll_tech_count to zero after
the call to nfc_targets_found().  When the race is
"lost" (i.e., ddev->poll_tech_count is found to not
be zero by the events started by nfc_targets_found()),
an error message is printed and the target is not found.
A similar race exists when digital_tg_recv_atr_req()
calls nfc_tm_activated().

Fix this by first saving the current value of
ddev->poll_tech_count and then clearing it before
calling nfc_targets_found()/nfc_tm_activated().
Clearing ddev->poll_tech_count before calling
nfc_targets_found()/nfc_tm_activated() eliminates
the race.  Saving the value is required so it can be
restored when nfc_targets_found()/nfc_tm_activated()
fails and polling needs to continue.

Acked-by: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

net/nfc/digital_core.c

@@ -299,6 +299,7 @@ int digital_target_found(struct nfc_digital_dev *ddev,
 	int rc;
 	u8 framing;
 	u8 rf_tech;
+	u8 poll_tech_count;
 	int (*check_crc)(struct sk_buff *skb);
 	void (*add_crc)(struct sk_buff *skb);
 
@@ -375,12 +376,16 @@ int digital_target_found(struct nfc_digital_dev *ddev,
 		return rc;
 
 	target->supported_protocols = (1 << protocol);
-	rc = nfc_targets_found(ddev->nfc_dev, target, 1);
-	if (rc)
-		return rc;
 
+	poll_tech_count = ddev->poll_tech_count;
 	ddev->poll_tech_count = 0;
 
+	rc = nfc_targets_found(ddev->nfc_dev, target, 1);
+	if (rc) {
+		ddev->poll_tech_count = poll_tech_count;
+		return rc;
+	}
+
 	return 0;
 }
 

net/nfc/digital_dep.c

@@ -673,6 +673,7 @@ void digital_tg_recv_atr_req(struct nfc_digital_dev *ddev, void *arg,
 	int rc;
 	struct digital_atr_req *atr_req;
 	size_t gb_len, min_size;
+	u8 poll_tech_count;
 
 	if (IS_ERR(resp)) {
 		rc = PTR_ERR(resp);
@@ -730,12 +731,16 @@ void digital_tg_recv_atr_req(struct nfc_digital_dev *ddev, void *arg,
 		goto exit;
 
 	gb_len = resp->len - sizeof(struct digital_atr_req);
+
+	poll_tech_count = ddev->poll_tech_count;
+	ddev->poll_tech_count = 0;
+
 	rc = nfc_tm_activated(ddev->nfc_dev, NFC_PROTO_NFC_DEP_MASK,
 			      NFC_COMM_PASSIVE, atr_req->gb, gb_len);
-	if (rc)
+	if (rc) {
+		ddev->poll_tech_count = poll_tech_count;
 		goto exit;
-
-	ddev->poll_tech_count = 0;
+	}
 
 	rc = 0;
 exit: