[PATCH] staging: pi433: add mutex fixing race condition when accessing tx_cfg

[Hugo Lefeuvre <hle@owl.eu.com>]

In the PI433_IOC_WR_TX_CFG case in pi433_ioctl, instance->tx_cfg is
modified using

copy_from_user(&instance->tx_cfg, argp, sizeof(struct pi433_tx_cfg)))

without any kind of synchronization. In the case where two threads
would execute this same command concurrently the tx_cfg field might
enter in an inconsistent state.

Add a mutex making sure that the PI433_IOC_WR_TX_CFG case will never
be run by several threads concurrently.

Signed-off-by: Hugo Lefeuvre <hle@owl.eu.com>
---
 drivers/staging/pi433/pi433_if.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/pi433/pi433_if.c b/drivers/staging/pi433/pi433_if.c
index d1e0ddbc79ce..94c9d5482f44 100644
--- a/drivers/staging/pi433/pi433_if.c
+++ b/drivers/staging/pi433/pi433_if.c
@@ -115,6 +115,7 @@ struct pi433_device {
 
 struct pi433_instance {
 	struct pi433_device	*device;
+	struct mutex		tx_cfg_lock; /* guards race conditions when updating tx config */
 	struct pi433_tx_cfg	tx_cfg;
 };
 
@@ -889,9 +890,13 @@ pi433_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 			return -EFAULT;
 		break;
 	case PI433_IOC_WR_TX_CFG:
+		mutex_lock(&instance->tx_cfg_lock);
 		if (copy_from_user(&instance->tx_cfg, argp,
-				   sizeof(struct pi433_tx_cfg)))
+				   sizeof(struct pi433_tx_cfg))) {
+			mutex_unlock(&instance->tx_cfg_lock);
 			return -EFAULT;
+		}
+		mutex_unlock(&instance->tx_cfg_lock);
 		break;
 	case PI433_IOC_RD_RX_CFG:
 		if (copy_to_user(argp, &device->rx_cfg,
@@ -966,6 +971,8 @@ static int pi433_open(struct inode *inode, struct file *filp)
 	instance->tx_cfg.bit_rate = 4711;
 	// TODO: fill instance->tx_cfg;
 
+	mutex_init(&instance->tx_cfg_lock);
+
 	/* instance data as context */
 	filp->private_data = instance;
 	nonseekable_open(inode, filp);
-- 
2.17.1