[PATCH] staging: pi433: fix race condition in pi433_open

[Hugo Lefeuvre]

Whenever pi433_open and pi433_remove execute concurrently, a race
condition potentially resulting in use-after-free might happen.

Let T1 and T2 be two kernel threads.

1. T1 executes pi433_open and stops before "device->users++".
2. The pi433 device was removed inbetween, so T2 executes pi433_remove
   and frees device because the user count has not been incremented yet.
3. T1 executes "device->users++" (use-after-free).

This race condition happens because the check of minor number and
user count increment does not happen atomically.

Fix: Extend scope of minor_lock in pi433_open().

Signed-off-by: Hugo Lefeuvre <hle@xxxxxxxxxx>
---
 drivers/staging/pi433/pi433_if.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/pi433/pi433_if.c b/drivers/staging/pi433/pi433_if.c
index 94e0bfcec991..73c511249f7f 100644
--- a/drivers/staging/pi433/pi433_if.c
+++ b/drivers/staging/pi433/pi433_if.c
@@ -957,11 +957,13 @@ static int pi433_open(struct inode *inode, struct file *filp)
 
 	mutex_lock(&minor_lock);
 	device = idr_find(&pi433_idr, iminor(inode));
-	mutex_unlock(&minor_lock);
 	if (!device) {
+		mutex_unlock(&minor_lock);
 		pr_debug("device: minor %d unknown.\n", iminor(inode));
 		return -ENODEV;
 	}
+	device->users++;
+	mutex_unlock(&minor_lock);
 
 	if (!device->rx_buffer) {
 		device->rx_buffer = kmalloc(MAX_MSG_SIZE, GFP_KERNEL);
@@ -969,7 +971,6 @@ static int pi433_open(struct inode *inode, struct file *filp)
 			return -ENOMEM;
 	}
 
-	device->users++;
 	instance = kzalloc(sizeof(*instance), GFP_KERNEL);
 	if (!instance) {
 		kfree(device->rx_buffer);
-- 
2.17.1