[PATCH] drivers/media/rc: Fix a race condition in send_packet()

[Gautam Menghani <gautammenghani201@gmail.com>]

The function send_packet() has a race condition as follows:
func send_packet()
{
    // do work
    call usb_submit_urb()
    mutex_unlock()
    wait_for_event_interruptible()  <-- lock gone
    mutex_lock()
}

func vfd_write()
{
    mutex_lock()
    call send_packet()  <- prev call is not completed
    mutex_unlock()
}

When the mutex is unlocked and the function send_packet() waits for the
call to complete, vfd_write() can start another call, which leads to the
"URB submitted while active" warning in usb_submit_urb(). 
Fix this by removing the mutex_unlock() call in send_packet() and using
mutex_lock_interruptible().

Fixes: 21677cfc562a ("V4L/DVB: ir-core: add imon driver")
Link: https://syzkaller.appspot.com/bug?id=e378e6a51fbe6c5cc43e34f131cc9a315ef0337e
Reported-by: syzbot+0c3cb6dc05fbbdc3ad66@syzkaller.appspotmail.com

Signed-off-by: Gautam Menghani <gautammenghani201@gmail.com>
---
 drivers/media/rc/imon.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/media/rc/imon.c b/drivers/media/rc/imon.c
index 735b925da998..91d8056666ec 100644
--- a/drivers/media/rc/imon.c
+++ b/drivers/media/rc/imon.c
@@ -646,15 +646,14 @@ static int send_packet(struct imon_context *ictx)
 		pr_err_ratelimited("error submitting urb(%d)\n", retval);
 	} else {
 		/* Wait for transmission to complete (or abort) */
-		mutex_unlock(&ictx->lock);
 		retval = wait_for_completion_interruptible(
 				&ictx->tx.finished);
 		if (retval) {
 			usb_kill_urb(ictx->tx_urb);
 			pr_err_ratelimited("task interrupted\n");
 		}
-		mutex_lock(&ictx->lock);
 
+		ictx->tx.busy = false;
 		retval = ictx->tx.status;
 		if (retval)
 			pr_err_ratelimited("packet tx failed (%d)\n", retval);
@@ -955,7 +954,8 @@ static ssize_t vfd_write(struct file *file, const char __user *buf,
 	if (ictx->disconnected)
 		return -ENODEV;
 
-	mutex_lock(&ictx->lock);
+	if (mutex_lock_interruptible(&ictx->lock))
+		return -ERESTARTSYS;
 
 	if (!ictx->dev_present_intf0) {
 		pr_err_ratelimited("no iMON device present\n");
-- 
2.34.1