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>

1 files changed, 8 insertions, 3 deletions

diff --git a/net/nfc/digital_core.c b/net/nfc/digital_core.c
index a6ce3c627e4e..361bc37d2db1 100644
--- a/net/nfc/digital_core.c
+++ b/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;
 }