staging: pi433: fix race condition in pi433_ioctl

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

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.

Additionally: if ioctl(PI433_IOC_WR_TX_CFG) and write() execute
concurrently the tx config might be modified while it is being
copied to the fifo, resulting in potential data corruption.

Fix: Get instance->tx_cfg_lock before modifying tx config in the
PI433_IOC_WR_TX_CFG case in pi433_ioctl.

Also, do not copy data directly from user space to instance->tx_cfg.
Instead use a temporary buffer allowing future checks for correctness
of copied data and simpler code.
Signed-off-by: Hugo Lefeuvre <hle@owl.eu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/pi433/pi433_if.c

@@ -880,6 +880,7 @@ pi433_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 	int			retval = 0;
 	struct pi433_instance	*instance;
 	struct pi433_device	*device;
+	struct pi433_tx_cfg	tx_cfg;
 	void __user *argp = (void __user *)arg;
 
 	/* Check type and command number */
@@ -902,9 +903,11 @@ pi433_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 			return -EFAULT;
 		break;
 	case PI433_IOC_WR_TX_CFG:
-		if (copy_from_user(&instance->tx_cfg, argp,
-				   sizeof(struct pi433_tx_cfg)))
+		if (copy_from_user(&tx_cfg, argp, sizeof(struct pi433_tx_cfg)))
 			return -EFAULT;
+		mutex_lock(&device->tx_fifo_lock);
+		memcpy(&instance->tx_cfg, &tx_cfg, sizeof(struct pi433_tx_cfg));
+		mutex_unlock(&device->tx_fifo_lock);
 		break;
 	case PI433_IOC_RD_RX_CFG:
 		if (copy_to_user(argp, &device->rx_cfg,