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Date:   Sun, 18 Sep 2022 21:04:57 -0700
From:   Hyunwoo Kim <imv4bel@gmail.com>
To:     laforge@gnumonks.org
Cc:     linux-kernel@vger.kernel.org, imv4bel@gmail.com, arnd@arndb.de,
        gregkh@linuxfoundation.org, linux@dominikbrodowski.net
Subject: [PATCH v3] char: pcmcia: cm4040_cs: Fix use-after-free in reader_fops
Message-ID: <20220919040457.GA302681@ubuntu>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Precedence: bulk

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 cm4040_open() function and the
reader_detach() function, which may eventually result in UAF.

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

Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
 drivers/char/pcmcia/cm4040_cs.c | 50 +++++++++++++++++++++++----------
 1 file changed, 35 insertions(+), 15 deletions(-)

diff --git a/drivers/char/pcmcia/cm4040_cs.c b/drivers/char/pcmcia/cm4040_cs.c
index 827711911da4..bb9116d81890 100644
--- a/drivers/char/pcmcia/cm4040_cs.c
+++ b/drivers/char/pcmcia/cm4040_cs.c
@@ -59,6 +59,7 @@ static DEFINE_MUTEX(cm4040_mutex);
 /* how often to poll for fifo status change */
 #define POLL_PERIOD 				msecs_to_jiffies(10)
 
+static void cm4040_delete(struct kref *kref);
 static void reader_release(struct pcmcia_device *link);
 
 static int major;
@@ -73,6 +74,7 @@ struct reader_dev {
 	wait_queue_head_t	poll_wait;
 	wait_queue_head_t	read_wait;
 	wait_queue_head_t	write_wait;
+	struct kref		refcnt;
 	unsigned long 	  	buffer_status;
 	unsigned long     	timeout;
 	unsigned char     	s_buf[READ_WRITE_BUFFER_SIZE];
@@ -102,6 +104,28 @@ static inline unsigned char xinb(unsigned short port)
 }
 #endif
 
+static void cm4040_delete(struct kref *kref)
+{
+	struct reader_dev *dev = container_of(kref, struct reader_dev, refcnt);
+	struct pcmcia_device *link = dev->p_dev;
+	int devno;
+
+	/* find device */
+	for (devno = 0; devno < CM_MAX_DEV; devno++) {
+		if (dev_table[devno] == link)
+			break;
+	}
+	if (devno == CM_MAX_DEV)
+		return;
+
+	reader_release(link);
+
+	dev_table[devno] = NULL;
+	kfree(dev);
+
+	device_destroy(cmx_class, MKDEV(major, devno));
+}
+
 /* poll the device fifo status register.  not to be confused with
  * the poll syscall. */
 static void cm4040_do_poll(struct timer_list *t)
@@ -442,6 +466,7 @@ static int cm4040_open(struct inode *inode, struct file *filp)
 		return -ENODEV;
 
 	mutex_lock(&cm4040_mutex);
+
 	link = dev_table[minor];
 	if (link == NULL || !pcmcia_dev_present(link)) {
 		ret = -ENODEV;
@@ -468,8 +493,11 @@ static int cm4040_open(struct inode *inode, struct file *filp)
 
 	DEBUGP(2, dev, "<- cm4040_open (successfully)\n");
 	ret = nonseekable_open(inode, filp);
+
+	kref_get(&dev->refcnt);
 out:
 	mutex_unlock(&cm4040_mutex);
+
 	return ret;
 }
 
@@ -495,6 +523,9 @@ static int cm4040_close(struct inode *inode, struct file *filp)
 	wake_up(&dev->devq);
 
 	DEBUGP(2, dev, "<- cm4040_close\n");
+
+	kref_put(&dev->refcnt, cm4040_delete);
+
 	return 0;
 }
 
@@ -584,6 +615,7 @@ static int reader_probe(struct pcmcia_device *link)
 	init_waitqueue_head(&dev->read_wait);
 	init_waitqueue_head(&dev->write_wait);
 	timer_setup(&dev->poll_timer, cm4040_do_poll, 0);
+	kref_init(&dev->refcnt);
 
 	ret = reader_config(link, i);
 	if (ret) {
@@ -600,22 +632,10 @@ static int reader_probe(struct pcmcia_device *link)
 static void reader_detach(struct pcmcia_device *link)
 {
 	struct reader_dev *dev = link->priv;
-	int devno;
-
-	/* find device */
-	for (devno = 0; devno < CM_MAX_DEV; devno++) {
-		if (dev_table[devno] == link)
-			break;
-	}
-	if (devno == CM_MAX_DEV)
-		return;
-
-	reader_release(link);
-
-	dev_table[devno] = NULL;
-	kfree(dev);
 
-	device_destroy(cmx_class, MKDEV(major, devno));
+	mutex_lock(&cm4040_mutex);
+	kref_put(&dev->refcnt, cm4040_delete);
+	mutex_unlock(&cm4040_mutex);
 
 	return;
 }
-- 
2.25.1

Dear,


I fixed the wrong patch referencing "dev" after kref_put() in the previous version of the patch.


Regards,
Hyunwoo Kim.