wimax i2400m: fix race condition while accessing rx_roq by using kref count

This patch fixes the race condition when one thread tries to destroy
the memory allocated for rx_roq, while another thread still happen
to access rx_roq.
Such a race condition occurs when i2400m-sdio kernel module gets
unloaded, destroying the memory allocated for rx_roq while rx_roq
is accessed by i2400m_rx_edata(), as explained below:
$thread1                                $thread2
$ void i2400m_rx_edata()                $
$Access rx_roq[]                        $
$roq = &i2400m->rx_roq[ro_cin]          $
$ i2400m_roq_[reset/queue/update_ws]    $
$                                       $ void i2400m_rx_release();
$                                       $kfree(rx->roq);
$                                       $rx->roq = NULL;
$Oops! rx_roq is NULL

This patch fixes the race condition using refcount approach.

Signed-off-by: Prasanna S. Panchamukhi <prasannax.s.panchamukhi@intel.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/drivers/net/wimax/i2400m/i2400m.h b/drivers/net/wimax/i2400m/i2400m.h
index 7a9c2c5b25cb..b8c7dbfead49 100644
--- a/drivers/net/wimax/i2400m/i2400m.h
+++ b/drivers/net/wimax/i2400m/i2400m.h
@@ -412,7 +412,7 @@ struct i2400m_barker_db;
  *
  * @tx_size_max: biggest TX message sent.
  *
- * @rx_lock: spinlock to protect RX members
+ * @rx_lock: spinlock to protect RX members and rx_roq_refcount.
  *
  * @rx_pl_num: total number of payloads received
  *
@@ -436,6 +436,10 @@ struct i2400m_barker_db;
  *     delivered. Then the driver can release them to the host. See
  *     drivers/net/i2400m/rx.c for details.
  *
+ * @rx_roq_refcount: refcount rx_roq. This refcounts any access to
+ *     rx_roq thus preventing rx_roq being destroyed when rx_roq
+ *     is being accessed. rx_roq_refcount is protected by rx_lock.
+ *
  * @rx_reports: reports received from the device that couldn't be
  *     processed because the driver wasn't still ready; when ready,
  *     they are pulled from here and chewed.
@@ -597,10 +601,12 @@ struct i2400m {
                 tx_num, tx_size_acc, tx_size_min, tx_size_max;
 
         /* RX stuff */
-        spinlock_t rx_lock;             /* protect RX state */
+        /* protect RX state and rx_roq_refcount */
+        spinlock_t rx_lock;
         unsigned rx_pl_num, rx_pl_max, rx_pl_min,
                 rx_num, rx_size_acc, rx_size_min, rx_size_max;
-        struct i2400m_roq *rx_roq;      /* not under rx_lock! */
+        struct i2400m_roq *rx_roq;      /* access is refcounted */
+        struct kref rx_roq_refcount;    /* refcount access to rx_roq */
         u8 src_mac_addr[ETH_HLEN];
         struct list_head rx_reports;    /* under rx_lock! */
         struct work_struct rx_report_ws;