rpmsg: char: Add lock to avoid race when rpmsg device is released

When remote host goes down glink char device channel is freed and
associated rpdev is destroyed through rpmsg_chrdev_eptdev_destroy(),
At the same time user space apps can still try to open/poll rpmsg
char device which will result in calling rpmsg_create_ept()/rpmsg_poll().
These functions will try to reference rpdev which has already been freed
through rpmsg_chrdev_eptdev_destroy().

File operation functions and device removal function must be protected
with lock. This patch adds existing ept lock in remove function as well.
Signed-off-by: Deepak Kumar Singh <quic_deesin@quicinc.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/1663584840-15762-2-git-send-email-quic_deesin@quicinc.com

drivers/rpmsg/rpmsg_char.c

@@ -75,6 +75,7 @@ int rpmsg_chrdev_eptdev_destroy(struct device *dev, void *data)
 	struct rpmsg_eptdev *eptdev = dev_to_eptdev(dev);
 
 	mutex_lock(&eptdev->ept_lock);
+	eptdev->rpdev = NULL;
 	if (eptdev->ept) {
 		/* The default endpoint is released by the rpmsg core */
 		if (!eptdev->default_ept)
@@ -128,6 +129,11 @@ static int rpmsg_eptdev_open(struct inode *inode, struct file *filp)
 		return -EBUSY;
 	}
 
+	if (!eptdev->rpdev) {
+		mutex_unlock(&eptdev->ept_lock);
+		return -ENETRESET;
+	}
+
 	get_device(dev);
 
 	/*
@@ -279,7 +285,9 @@ static __poll_t rpmsg_eptdev_poll(struct file *filp, poll_table *wait)
 	if (!skb_queue_empty(&eptdev->queue))
 		mask |= EPOLLIN | EPOLLRDNORM;
 
+	mutex_lock(&eptdev->ept_lock);
 	mask |= rpmsg_poll(eptdev->ept, filp, wait);
+	mutex_unlock(&eptdev->ept_lock);
 
 	return mask;
 }