1 files changed, 31 insertions, 7 deletions

diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 4ee61e0ebd7e..83611f38ec13 100644
--- a/drivers/hid/uhid.c
+++ b/drivers/hid/uhid.c
@@ -26,6 +26,7 @@
 #include <linux/uhid.h>
 #include <linux/wait.h>
 #include <linux/uaccess.h>
+#include <linux/eventpoll.h>
 
 #define UHID_NAME	"uhid"
 #define UHID_BUFSIZE	32
@@ -34,11 +35,22 @@ static DEFINE_MUTEX(uhid_open_mutex);
 
 struct uhid_device {
 	struct mutex devlock;
+
+	/* This flag tracks whether the HID device is usable for commands from
+	 * userspace. The flag is already set before hid_add_device(), which
+	 * runs in workqueue context, to allow hid_add_device() to communicate
+	 * with userspace.
+	 * However, if hid_add_device() fails, the flag is cleared without
+	 * holding devlock.
+	 * We guarantee that if @running changes from true to false while you're
+	 * holding @devlock, it's still fine to access @hid.
+	 */
 	bool running;
 
 	__u8 *rd_data;
 	uint rd_size;
 
+	/* When this is NULL, userspace may use UHID_CREATE/UHID_CREATE2. */
 	struct hid_device *hid;
 	struct uhid_event input_buf;
 
@@ -69,9 +81,18 @@ static void uhid_device_add_worker(struct work_struct *work)
 	if (ret) {
 		hid_err(uhid->hid, "Cannot register HID device: error %d\n", ret);
 
-		hid_destroy_device(uhid->hid);
-		uhid->hid = NULL;
+		/* We used to call hid_destroy_device() here, but that's really
+		 * messy to get right because we have to coordinate with
+		 * concurrent writes from userspace that might be in the middle
+		 * of using uhid->hid.
+		 * Just leave uhid->hid as-is for now, and clean it up when
+		 * userspace tries to close or reinitialize the uhid instance.
+		 *
+		 * However, we do have to clear the ->running flag and do a
+		 * wakeup to make sure userspace knows that the device is gone.
+		 */
 		uhid->running = false;
+		wake_up_interruptible(&uhid->report_wait);
 	}
 }
 
@@ -384,7 +405,7 @@ static int uhid_hid_output_report(struct hid_device *hid, __u8 *buf,
 	return uhid_hid_output_raw(hid, buf, count, HID_OUTPUT_REPORT);
 }
 
-static struct hid_ll_driver uhid_hid_driver = {
+struct hid_ll_driver uhid_hid_driver = {
 	.start = uhid_hid_start,
 	.stop = uhid_hid_stop,
 	.open = uhid_hid_open,
@@ -393,6 +414,7 @@ static struct hid_ll_driver uhid_hid_driver = {
 	.raw_request = uhid_hid_raw_request,
 	.output_report = uhid_hid_output_report,
 };
+EXPORT_SYMBOL_GPL(uhid_hid_driver);
 
 #ifdef CONFIG_COMPAT
 
@@ -490,7 +512,7 @@ static int uhid_dev_create2(struct uhid_device *uhid,
 	void *rd_data;
 	int ret;
 
-	if (uhid->running)
+	if (uhid->hid)
 		return -EALREADY;
 
 	rd_size = ev->u.create2.rd_size;
@@ -571,7 +593,7 @@ static int uhid_dev_create(struct uhid_device *uhid,
 
 static int uhid_dev_destroy(struct uhid_device *uhid)
 {
-	if (!uhid->running)
+	if (!uhid->hid)
 		return -EINVAL;
 
 	uhid->running = false;
@@ -580,6 +602,7 @@ static int uhid_dev_destroy(struct uhid_device *uhid)
 	cancel_work_sync(&uhid->worker);
 
 	hid_destroy_device(uhid->hid);
+	uhid->hid = NULL;
 	kfree(uhid->rd_data);
 
 	return 0;
@@ -781,13 +804,14 @@ unlock:
 static unsigned int uhid_char_poll(struct file *file, poll_table *wait)
 {
 	struct uhid_device *uhid = file->private_data;
+	unsigned int mask = POLLOUT | POLLWRNORM; /* uhid is always writable */
 
 	poll_wait(file, &uhid->waitq, wait);
 
 	if (uhid->head != uhid->tail)
-		return POLLIN | POLLRDNORM;
+		mask |= POLLIN | POLLRDNORM;
 
-	return 0;
+	return mask;
 }
 
 static const struct file_operations uhid_fops = {