[PATCH v3] char: pcmcia: cm4040_cs: Fix use-after-free in reader_fops

From: Hyunwoo Kim
Date: Mon Sep 19 2022 - 00:05:09 EST


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@xxxxxxxxx>
---
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.