[PATCH] char: pcmcia: cm4000_cs: Fix use-after-free in cm4000_fops

[Hyunwoo Kim]

A race condition may occur if the user physically removes the pcmcia
device while calling open() for this char device node.

This is a race condition between the cmm_open() function and the
cm4000_detach() function, which may eventually result in UAF.

So, add a refcount check to cm4000_detach() to free the "dev" structure
after the char device node is close()d.

Signed-off-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
---
 drivers/char/pcmcia/cm4000_cs.c | 73 ++++++++++++++++++++++-----------
 1 file changed, 50 insertions(+), 23 deletions(-)

diff --git a/drivers/char/pcmcia/cm4000_cs.c b/drivers/char/pcmcia/cm4000_cs.c
index adaec8fd4b16..3cb0342189d3 100644
--- a/drivers/char/pcmcia/cm4000_cs.c
+++ b/drivers/char/pcmcia/cm4000_cs.c
@@ -104,6 +104,8 @@ static int major;		/* major number we get from the kernel */
 
 struct cm4000_dev {
 	struct pcmcia_device *p_dev;
+	struct mutex		lock;
+	struct kref		refcnt;
 
 	unsigned char atr[MAX_ATR];
 	unsigned char rbuf[512];
@@ -146,6 +148,9 @@ struct cm4000_dev {
 
 #define	ZERO_DEV(dev)	memset(&((dev)->init), 0, sizeof((dev)->init))
 
+static void stop_monitor(struct cm4000_dev *dev);
+static void cm4000_delete(struct kref *kref);
+
 static struct pcmcia_device *dev_table[CM4000_MAX_DEV];
 static struct class *cmm_class;
 
@@ -416,6 +421,30 @@ static struct card_fixup card_fixups[] = {
 	},
 };
 
+
+static void cm4000_delete(struct kref *kref)
+{
+	struct cm4000_dev *dev = container_of(kref, struct cm4000_dev, refcnt);
+	struct pcmcia_device *link = dev->p_dev;
+	int devno;
+
+	/* find device */
+	for (devno = 0; devno < CM4000_MAX_DEV; devno++)
+		if (dev_table[devno] == link)
+			break;
+	if (devno == CM4000_MAX_DEV)
+		return;
+
+	stop_monitor(dev);
+
+	cm4000_release(link);
+
+	dev_table[devno] = NULL;
+	kfree(dev);
+
+	device_destroy(cmm_class, MKDEV(major, devno));
+}
+
 static void set_cardparameter(struct cm4000_dev *dev)
 {
 	int i;
@@ -1632,16 +1661,18 @@ static int cmm_open(struct inode *inode, struct file *filp)
 	mutex_lock(&cmm_mutex);
 	link = dev_table[minor];
 	if (link == NULL || !pcmcia_dev_present(link)) {
-		ret = -ENODEV;
-		goto out;
+		mutex_unlock(&cmm_mutex);
+		return -ENODEV;
 	}
 
 	if (link->open) {
-		ret = -EBUSY;
-		goto out;
+		mutex_unlock(&cmm_mutex);
+		return -EBUSY;
 	}
 
 	dev = link->priv;
+	mutex_lock(&dev->lock);
+
 	filp->private_data = dev;
 
 	DEBUGP(2, dev, "-> cmm_open(device=%d.%d process=%s,%d)\n",
@@ -1660,8 +1691,9 @@ static int cmm_open(struct inode *inode, struct file *filp)
 	 * inserted)
 	 */
 	if (filp->f_flags & O_NONBLOCK) {
-		ret = -EAGAIN;
-		goto out;
+		mutex_unlock(&cmm_mutex);
+		mutex_unlock(&dev->lock);
+		return -EAGAIN;
 	}
 
 	dev->mdelay = T_50MSEC;
@@ -1673,8 +1705,12 @@ static int cmm_open(struct inode *inode, struct file *filp)
 
 	DEBUGP(2, dev, "<- cmm_open\n");
 	ret = stream_open(inode, filp);
-out:
+
+	kref_get(&dev->refcnt);
+
 	mutex_unlock(&cmm_mutex);
+	mutex_unlock(&dev->lock);
+
 	return ret;
 }
 
@@ -1703,6 +1739,8 @@ static int cmm_close(struct inode *inode, struct file *filp)
 	link->open = 0;		/* only one open per device */
 	wake_up(&dev->devq);	/* socket removed? */
 
+	kref_put(&dev->refcnt, cm4000_delete);
+
 	DEBUGP(2, dev, "cmm_close\n");
 	return 0;
 }
@@ -1808,6 +1846,8 @@ static int cm4000_probe(struct pcmcia_device *link)
 	init_waitqueue_head(&dev->ioq);
 	init_waitqueue_head(&dev->atrq);
 	init_waitqueue_head(&dev->readq);
+	kref_init(&dev->refcnt);
+	mutex_init(&dev->lock);
 
 	ret = cm4000_config(link, i);
 	if (ret) {
@@ -1824,23 +1864,10 @@ static int cm4000_probe(struct pcmcia_device *link)
 static void cm4000_detach(struct pcmcia_device *link)
 {
 	struct cm4000_dev *dev = link->priv;
-	int devno;
-
-	/* find device */
-	for (devno = 0; devno < CM4000_MAX_DEV; devno++)
-		if (dev_table[devno] == link)
-			break;
-	if (devno == CM4000_MAX_DEV)
-		return;
-
-	stop_monitor(dev);
 
-	cm4000_release(link);
-
-	dev_table[devno] = NULL;
-	kfree(dev);
-
-	device_destroy(cmm_class, MKDEV(major, devno));
+	mutex_lock(&dev->lock);
+	kref_put(&dev->refcnt, cm4000_delete);
+	mutex_unlock(&dev->lock);
 
 	return;
 }
-- 
2.25.1