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Subject: [PATCH] usb: gadgetfs: fix use-after-free in ep_open()
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci


ep_open() dereferenced inode->i_private and locked its embedded
mutex before checking whether the underlying ep_data was still
alive. destroy_ep_files(), called from gadgetfs_unbind(), can free
that same ep_data concurrently, since nothing prevented an in-flight
open() from racing the teardown. KASAN reports a slab-use-after-free
read inside __mutex_lock_common() from ep_open(), with the object
freed by a concurrent gadgetfs_unbind() -> destroy_ep_files() ->
put_ep() -> kfree().

This race was identified by Al Viro on LKML in 2006 ("races in
drivers/usb/gadget/inode.c") but never fixed in ep_open() itself.

Fix it by checking dev->state and reading inode->i_private only
under dev->lock, and pinning the result with get_ep() before
dev->lock is dropped and before data->lock is touched. dev is
reached via the file-scope singleton the_device, which stays valid
for the duration of any open() in progress, so no dereference of
ep_data happens before its liveness is confirmed.

This depends on dev->state being set to STATE_DEV_UNBOUND under
dev->lock before destroy_ep_files() is called. gadgetfs_unbind()
already does this; activate_ep_files()'s enomem0 cleanup path did
not, leaving the same race open on the bind-failure path. This patch
adds the missing state transition there too.

Reported-by: syzbot+df9e891bf8ea586f846b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=df9e891bf8ea586f846b
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 drivers/usb/gadget/legacy/inode.c | 40 +++++++++++++++++++++----------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/drivers/usb/gadget/legacy/inode.c b/drivers/usb/gadget/legacy/inode.c
index d87a8ab51510..0d0989d79020 100644
--- a/drivers/usb/gadget/legacy/inode.c
+++ b/drivers/usb/gadget/legacy/inode.c
@@ -235,6 +235,7 @@ static void put_ep (struct ep_data *data)
 
 static const char *CHIP;
 static DEFINE_MUTEX(sb_mutex);		/* Serialize superblock operations */
+static struct dev_data *the_device;
 
 /*----------------------------------------------------------------------*/
 
@@ -817,25 +818,39 @@ ep_config (struct ep_data *data, const char *buf, size_t len)
 static int
 ep_open (struct inode *inode, struct file *fd)
 {
-	struct ep_data		*data = inode->i_private;
-	int			value = -EBUSY;
+	struct dev_data *dev = the_device;
+	struct ep_data *data;
+	int value = -ENODEV;
+
+	spin_lock_irq (&dev->lock);
+	if (dev->state == STATE_DEV_UNBOUND) {
+		spin_unlock_irq (&dev->lock);
+		return -ENOENT;
+	}
+	data = inode->i_private;
+	get_ep (data);
+	spin_unlock_irq (&dev->lock);
 
-	if (mutex_lock_interruptible(&data->lock) != 0)
+	if (mutex_lock_interruptible(&data->lock) != 0) {
+		put_ep (data);
 		return -EINTR;
-	spin_lock_irq (&data->dev->lock);
-	if (data->dev->state == STATE_DEV_UNBOUND)
+	}
+
+	value = -EBUSY;
+	spin_lock_irq (&dev->lock);
+	if (dev->state == STATE_DEV_UNBOUND)
 		value = -ENOENT;
 	else if (data->state == STATE_EP_DISABLED) {
 		value = 0;
 		data->state = STATE_EP_READY;
-		get_ep (data);
 		fd->private_data = data;
-		VDEBUG (data->dev, "%s ready\n", data->name);
+		VDEBUG (dev, "%s ready\n", data->name);
 	} else
-		DBG (data->dev, "%s state %d\n",
-			data->name, data->state);
-	spin_unlock_irq (&data->dev->lock);
+		DBG (dev, "%s state %d\n", data->name, data->state);
+	spin_unlock_irq (&dev->lock);
 	mutex_unlock(&data->lock);
+	if (value)
+		put_ep (data);
 	return value;
 }
 
@@ -1632,6 +1647,9 @@ static int activate_ep_files (struct dev_data *dev)
 	kfree (data);
 enomem0:
 	DBG (dev, "%s enomem\n", __func__);
+	spin_lock_irq (&dev->lock);
+	dev->state = STATE_DEV_UNBOUND;
+	spin_unlock_irq (&dev->lock);
 	destroy_ep_files (dev);
 	return -ENOMEM;
 }
@@ -1663,8 +1681,6 @@ gadgetfs_unbind (struct usb_gadget *gadget)
 	put_dev (dev);
 }
 
-static struct dev_data		*the_device;
-
 static int gadgetfs_bind(struct usb_gadget *gadget,
 		struct usb_gadget_driver *driver)
 {
-- 
2.43.0