[PATCH 0/4] USB: usblcd: disconnect fix and locking clean ups

[PATCH 0/4] USB: usblcd: disconnect fix and locking clean ups

[Johan Hovold]

This series fixes a failure to stop I/O on disconnect() in the usblcd
driver. Turns out there was a lot of legacy cruft in this driver which
could simply be removed.

The first patch is marked for stable and could go into v5.4 while the
rest is v5.5 material. Posting all at once for completeness.

I was tempted to rip out the custom ioctls() used to retrieve the driver
version and bcdDevice (sic!), but decided to leave them in. I doubt
anyone would miss them though so perhaps we should give it a go?

Tested using a mockup device.

Johan


Johan Hovold (4):
  USB: usblcd: fix I/O after disconnect
  USB: usblcd: drop redundant disconnect mutex
  USB: usblcd: drop redundant lcd mutex
  USB: usblcd: use pr_err()

 drivers/usb/misc/usblcd.c | 60 +++++++++++++++++++++------------------
 1 file changed, 33 insertions(+), 27 deletions(-)

-- 
2.23.0

[PATCH 1/4] USB: usblcd: fix I/O after disconnect

[Johan Hovold]

Make sure to stop all I/O on disconnect by adding a disconnected flag
which is used to prevent new I/O from being started and by stopping all
ongoing I/O before returning.

This also fixes a potential use-after-free on driver unbind in case the
driver data is freed before the completion handler has run.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>	# 7bbe990c989e
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/usblcd.c | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/misc/usblcd.c b/drivers/usb/misc/usblcd.c
index 9ba4a4e68d91..aa982d3ca36b 100644
--- a/drivers/usb/misc/usblcd.c
+++ b/drivers/usb/misc/usblcd.c
@@ -18,6 +18,7 @@
 #include <linux/slab.h>
 #include <linux/errno.h>
 #include <linux/mutex.h>
+#include <linux/rwsem.h>
 #include <linux/uaccess.h>
 #include <linux/usb.h>
 
@@ -57,6 +58,8 @@ struct usb_lcd {
 							   using up all RAM */
 	struct usb_anchor	submitted;		/* URBs to wait for
 							   before suspend */
+	struct rw_semaphore	io_rwsem;
+	unsigned long		disconnected:1;
 };
 #define to_lcd_dev(d) container_of(d, struct usb_lcd, kref)
 
@@ -142,6 +145,13 @@ static ssize_t lcd_read(struct file *file, char __user * buffer,
 
 	dev = file->private_data;
 
+	down_read(&dev->io_rwsem);
+
+	if (dev->disconnected) {
+		retval = -ENODEV;
+		goto out_up_io;
+	}
+
 	/* do a blocking bulk read to get data from the device */
 	retval = usb_bulk_msg(dev->udev,
 			      usb_rcvbulkpipe(dev->udev,
@@ -158,6 +168,9 @@ static ssize_t lcd_read(struct file *file, char __user * buffer,
 			retval = bytes_read;
 	}
 
+out_up_io:
+	up_read(&dev->io_rwsem);
+
 	return retval;
 }
 
@@ -237,11 +250,18 @@ static ssize_t lcd_write(struct file *file, const char __user * user_buffer,
 	if (r < 0)
 		return -EINTR;
 
+	down_read(&dev->io_rwsem);
+
+	if (dev->disconnected) {
+		retval = -ENODEV;
+		goto err_up_io;
+	}
+
 	/* create a urb, and a buffer for it, and copy the data to the urb */
 	urb = usb_alloc_urb(0, GFP_KERNEL);
 	if (!urb) {
 		retval = -ENOMEM;
-		goto err_no_buf;
+		goto err_up_io;
 	}
 
 	buf = usb_alloc_coherent(dev->udev, count, GFP_KERNEL,
@@ -278,6 +298,7 @@ static ssize_t lcd_write(struct file *file, const char __user * user_buffer,
 	   the USB core will eventually free it entirely */
 	usb_free_urb(urb);
 
+	up_read(&dev->io_rwsem);
 exit:
 	return count;
 error_unanchor:
@@ -285,7 +306,8 @@ static ssize_t lcd_write(struct file *file, const char __user * user_buffer,
 error:
 	usb_free_coherent(dev->udev, count, buf, urb->transfer_dma);
 	usb_free_urb(urb);
-err_no_buf:
+err_up_io:
+	up_read(&dev->io_rwsem);
 	up(&dev->limit_sem);
 	return retval;
 }
@@ -325,6 +347,7 @@ static int lcd_probe(struct usb_interface *interface,
 
 	kref_init(&dev->kref);
 	sema_init(&dev->limit_sem, USB_LCD_CONCURRENT_WRITES);
+	init_rwsem(&dev->io_rwsem);
 	init_usb_anchor(&dev->submitted);
 
 	dev->udev = usb_get_dev(interface_to_usbdev(interface));
@@ -422,6 +445,12 @@ static void lcd_disconnect(struct usb_interface *interface)
 	/* give back our minor */
 	usb_deregister_dev(interface, &lcd_class);
 
+	down_write(&dev->io_rwsem);
+	dev->disconnected = 1;
+	up_write(&dev->io_rwsem);
+
+	usb_kill_anchored_urbs(&dev->submitted);
+
 	/* decrement our usage count */
 	kref_put(&dev->kref, lcd_delete);
 
-- 
2.23.0

[PATCH 2/4] USB: usblcd: drop redundant disconnect mutex

[Johan Hovold]

Drop the redundant disconnect mutex which was introduced after the
open-disconnect race had been addressed generally in USB core by commit
d4ead16f50f9 ("USB: prevent char device open/deregister race").

Specifically, the rw-semaphore in core guarantees that all calls to
open() will have completed and that no new calls to open() will occur
after usb_deregister_dev() returns. Hence there is no need use the
driver data as an inverted disconnected flag.

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/usblcd.c | 18 +-----------------
 1 file changed, 1 insertion(+), 17 deletions(-)

diff --git a/drivers/usb/misc/usblcd.c b/drivers/usb/misc/usblcd.c
index aa982d3ca36b..b898650a5570 100644
--- a/drivers/usb/misc/usblcd.c
+++ b/drivers/usb/misc/usblcd.c
@@ -37,9 +37,6 @@ static const struct usb_device_id id_table[] = {
 };
 MODULE_DEVICE_TABLE(usb, id_table);
 
-static DEFINE_MUTEX(open_disc_mutex);
-
-
 struct usb_lcd {
 	struct usb_device	*udev;			/* init: probe_lcd */
 	struct usb_interface	*interface;		/* the interface for
@@ -95,17 +92,10 @@ static int lcd_open(struct inode *inode, struct file *file)
 		return -ENODEV;
 	}
 
-	mutex_lock(&open_disc_mutex);
 	dev = usb_get_intfdata(interface);
-	if (!dev) {
-		mutex_unlock(&open_disc_mutex);
-		mutex_unlock(&lcd_mutex);
-		return -ENODEV;
-	}
 
 	/* increment our usage count for the device */
 	kref_get(&dev->kref);
-	mutex_unlock(&open_disc_mutex);
 
 	/* grab a power reference */
 	r = usb_autopm_get_interface(interface);
@@ -388,7 +378,6 @@ static int lcd_probe(struct usb_interface *interface,
 		/* something prevented us from registering this driver */
 		dev_err(&interface->dev,
 			"Not able to get a minor for this device.\n");
-		usb_set_intfdata(interface, NULL);
 		goto error;
 	}
 
@@ -434,14 +423,9 @@ static int lcd_resume(struct usb_interface *intf)
 
 static void lcd_disconnect(struct usb_interface *interface)
 {
-	struct usb_lcd *dev;
+	struct usb_lcd *dev = usb_get_intfdata(interface);
 	int minor = interface->minor;
 
-	mutex_lock(&open_disc_mutex);
-	dev = usb_get_intfdata(interface);
-	usb_set_intfdata(interface, NULL);
-	mutex_unlock(&open_disc_mutex);
-
 	/* give back our minor */
 	usb_deregister_dev(interface, &lcd_class);
 
-- 
2.23.0

[PATCH 3/4] USB: usblcd: drop redundant lcd mutex

[Johan Hovold]

Drop the redundant lcd mutex introduced by commit 925ce689bb31 ("USB:
autoconvert trivial BKL users to private mutex") which replaced an
earlier BKL use.

The lock serialised calls to open() against other open() and a custom
ioctl() returning the bcdDevice (sic!), but neither is needed.

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/usblcd.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/drivers/usb/misc/usblcd.c b/drivers/usb/misc/usblcd.c
index b898650a5570..732eb1f81368 100644
--- a/drivers/usb/misc/usblcd.c
+++ b/drivers/usb/misc/usblcd.c
@@ -30,7 +30,6 @@
 #define IOCTL_GET_DRV_VERSION	2
 
 
-static DEFINE_MUTEX(lcd_mutex);
 static const struct usb_device_id id_table[] = {
 	{ .idVendor = 0x10D2, .match_flags = USB_DEVICE_ID_MATCH_VENDOR, },
 	{ },
@@ -81,12 +80,10 @@ static int lcd_open(struct inode *inode, struct file *file)
 	struct usb_interface *interface;
 	int subminor, r;
 
-	mutex_lock(&lcd_mutex);
 	subminor = iminor(inode);
 
 	interface = usb_find_interface(&lcd_driver, subminor);
 	if (!interface) {
-		mutex_unlock(&lcd_mutex);
 		printk(KERN_ERR "USBLCD: %s - error, can't find device for minor %d\n",
 		       __func__, subminor);
 		return -ENODEV;
@@ -101,13 +98,11 @@ static int lcd_open(struct inode *inode, struct file *file)
 	r = usb_autopm_get_interface(interface);
 	if (r < 0) {
 		kref_put(&dev->kref, lcd_delete);
-		mutex_unlock(&lcd_mutex);
 		return r;
 	}
 
 	/* save our object in the file's private structure */
 	file->private_data = dev;
-	mutex_unlock(&lcd_mutex);
 
 	return 0;
 }
@@ -176,14 +171,12 @@ static long lcd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 
 	switch (cmd) {
 	case IOCTL_GET_HARD_VERSION:
-		mutex_lock(&lcd_mutex);
 		bcdDevice = le16_to_cpu((dev->udev)->descriptor.bcdDevice);
 		sprintf(buf, "%1d%1d.%1d%1d",
 			(bcdDevice & 0xF000)>>12,
 			(bcdDevice & 0xF00)>>8,
 			(bcdDevice & 0xF0)>>4,
 			(bcdDevice & 0xF));
-		mutex_unlock(&lcd_mutex);
 		if (copy_to_user((void __user *)arg, buf, strlen(buf)) != 0)
 			return -EFAULT;
 		break;
-- 
2.23.0

[PATCH 4/4] USB: usblcd: use pr_err()

[Johan Hovold]

Replace the one remaining printk with pr_err().

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/usblcd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/misc/usblcd.c b/drivers/usb/misc/usblcd.c
index 732eb1f81368..61e9e987fe4a 100644
--- a/drivers/usb/misc/usblcd.c
+++ b/drivers/usb/misc/usblcd.c
@@ -84,7 +84,7 @@ static int lcd_open(struct inode *inode, struct file *file)
 
 	interface = usb_find_interface(&lcd_driver, subminor);
 	if (!interface) {
-		printk(KERN_ERR "USBLCD: %s - error, can't find device for minor %d\n",
+		pr_err("USBLCD: %s - error, can't find device for minor %d\n",
 		       __func__, subminor);
 		return -ENODEV;
 	}
-- 
2.23.0

[PATCH 0/4] USB: usblcd: disconnect fix and locking clean ups

[Johan Hovold]

This series fixes a failure to stop I/O on disconnect() in the usblcd
driver. Turns out there was a lot of legacy cruft in this driver which
could simply be removed.

The first patch is marked for stable and could go into v5.4 while the
rest is v5.5 material. Posting all at once for completeness.

I was tempted to rip out the custom ioctls() used to retrieve the driver
version and bcdDevice (sic!), but decided to leave them in. I doubt
anyone would miss them though so perhaps we should give it a go?

Tested using a mockup device.

Johan


Johan Hovold (4):
  USB: usblcd: fix I/O after disconnect
  USB: usblcd: drop redundant disconnect mutex
  USB: usblcd: drop redundant lcd mutex
  USB: usblcd: use pr_err()

 drivers/usb/misc/usblcd.c | 60 +++++++++++++++++++++------------------
 1 file changed, 33 insertions(+), 27 deletions(-)

-- 
2.23.0

[PATCH 1/4] USB: usblcd: fix I/O after disconnect

[Johan Hovold]

Make sure to stop all I/O on disconnect by adding a disconnected flag
which is used to prevent new I/O from being started and by stopping all
ongoing I/O before returning.

This also fixes a potential use-after-free on driver unbind in case the
driver data is freed before the completion handler has run.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>	# 7bbe990c989e
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/usblcd.c | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/misc/usblcd.c b/drivers/usb/misc/usblcd.c
index 9ba4a4e68d91..aa982d3ca36b 100644
--- a/drivers/usb/misc/usblcd.c
+++ b/drivers/usb/misc/usblcd.c
@@ -18,6 +18,7 @@
 #include <linux/slab.h>
 #include <linux/errno.h>
 #include <linux/mutex.h>
+#include <linux/rwsem.h>
 #include <linux/uaccess.h>
 #include <linux/usb.h>
 
@@ -57,6 +58,8 @@ struct usb_lcd {
 							   using up all RAM */
 	struct usb_anchor	submitted;		/* URBs to wait for
 							   before suspend */
+	struct rw_semaphore	io_rwsem;
+	unsigned long		disconnected:1;
 };
 #define to_lcd_dev(d) container_of(d, struct usb_lcd, kref)
 
@@ -142,6 +145,13 @@ static ssize_t lcd_read(struct file *file, char __user * buffer,
 
 	dev = file->private_data;
 
+	down_read(&dev->io_rwsem);
+
+	if (dev->disconnected) {
+		retval = -ENODEV;
+		goto out_up_io;
+	}
+
 	/* do a blocking bulk read to get data from the device */
 	retval = usb_bulk_msg(dev->udev,
 			      usb_rcvbulkpipe(dev->udev,
@@ -158,6 +168,9 @@ static ssize_t lcd_read(struct file *file, char __user * buffer,
 			retval = bytes_read;
 	}
 
+out_up_io:
+	up_read(&dev->io_rwsem);
+
 	return retval;
 }
 
@@ -237,11 +250,18 @@ static ssize_t lcd_write(struct file *file, const char __user * user_buffer,
 	if (r < 0)
 		return -EINTR;
 
+	down_read(&dev->io_rwsem);
+
+	if (dev->disconnected) {
+		retval = -ENODEV;
+		goto err_up_io;
+	}
+
 	/* create a urb, and a buffer for it, and copy the data to the urb */
 	urb = usb_alloc_urb(0, GFP_KERNEL);
 	if (!urb) {
 		retval = -ENOMEM;
-		goto err_no_buf;
+		goto err_up_io;
 	}
 
 	buf = usb_alloc_coherent(dev->udev, count, GFP_KERNEL,
@@ -278,6 +298,7 @@ static ssize_t lcd_write(struct file *file, const char __user * user_buffer,
 	   the USB core will eventually free it entirely */
 	usb_free_urb(urb);
 
+	up_read(&dev->io_rwsem);
 exit:
 	return count;
 error_unanchor:
@@ -285,7 +306,8 @@ static ssize_t lcd_write(struct file *file, const char __user * user_buffer,
 error:
 	usb_free_coherent(dev->udev, count, buf, urb->transfer_dma);
 	usb_free_urb(urb);
-err_no_buf:
+err_up_io:
+	up_read(&dev->io_rwsem);
 	up(&dev->limit_sem);
 	return retval;
 }
@@ -325,6 +347,7 @@ static int lcd_probe(struct usb_interface *interface,
 
 	kref_init(&dev->kref);
 	sema_init(&dev->limit_sem, USB_LCD_CONCURRENT_WRITES);
+	init_rwsem(&dev->io_rwsem);
 	init_usb_anchor(&dev->submitted);
 
 	dev->udev = usb_get_dev(interface_to_usbdev(interface));
@@ -422,6 +445,12 @@ static void lcd_disconnect(struct usb_interface *interface)
 	/* give back our minor */
 	usb_deregister_dev(interface, &lcd_class);
 
+	down_write(&dev->io_rwsem);
+	dev->disconnected = 1;
+	up_write(&dev->io_rwsem);
+
+	usb_kill_anchored_urbs(&dev->submitted);
+
 	/* decrement our usage count */
 	kref_put(&dev->kref, lcd_delete);
 
-- 
2.23.0

[PATCH 2/4] USB: usblcd: drop redundant disconnect mutex

[Johan Hovold]

Drop the redundant disconnect mutex which was introduced after the
open-disconnect race had been addressed generally in USB core by commit
d4ead16f50f9 ("USB: prevent char device open/deregister race").

Specifically, the rw-semaphore in core guarantees that all calls to
open() will have completed and that no new calls to open() will occur
after usb_deregister_dev() returns. Hence there is no need use the
driver data as an inverted disconnected flag.

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/usblcd.c | 18 +-----------------
 1 file changed, 1 insertion(+), 17 deletions(-)

diff --git a/drivers/usb/misc/usblcd.c b/drivers/usb/misc/usblcd.c
index aa982d3ca36b..b898650a5570 100644
--- a/drivers/usb/misc/usblcd.c
+++ b/drivers/usb/misc/usblcd.c
@@ -37,9 +37,6 @@ static const struct usb_device_id id_table[] = {
 };
 MODULE_DEVICE_TABLE(usb, id_table);
 
-static DEFINE_MUTEX(open_disc_mutex);
-
-
 struct usb_lcd {
 	struct usb_device	*udev;			/* init: probe_lcd */
 	struct usb_interface	*interface;		/* the interface for
@@ -95,17 +92,10 @@ static int lcd_open(struct inode *inode, struct file *file)
 		return -ENODEV;
 	}
 
-	mutex_lock(&open_disc_mutex);
 	dev = usb_get_intfdata(interface);
-	if (!dev) {
-		mutex_unlock(&open_disc_mutex);
-		mutex_unlock(&lcd_mutex);
-		return -ENODEV;
-	}
 
 	/* increment our usage count for the device */
 	kref_get(&dev->kref);
-	mutex_unlock(&open_disc_mutex);
 
 	/* grab a power reference */
 	r = usb_autopm_get_interface(interface);
@@ -388,7 +378,6 @@ static int lcd_probe(struct usb_interface *interface,
 		/* something prevented us from registering this driver */
 		dev_err(&interface->dev,
 			"Not able to get a minor for this device.\n");
-		usb_set_intfdata(interface, NULL);
 		goto error;
 	}
 
@@ -434,14 +423,9 @@ static int lcd_resume(struct usb_interface *intf)
 
 static void lcd_disconnect(struct usb_interface *interface)
 {
-	struct usb_lcd *dev;
+	struct usb_lcd *dev = usb_get_intfdata(interface);
 	int minor = interface->minor;
 
-	mutex_lock(&open_disc_mutex);
-	dev = usb_get_intfdata(interface);
-	usb_set_intfdata(interface, NULL);
-	mutex_unlock(&open_disc_mutex);
-
 	/* give back our minor */
 	usb_deregister_dev(interface, &lcd_class);
 
-- 
2.23.0

[PATCH 3/4] USB: usblcd: drop redundant lcd mutex

[Johan Hovold]

Drop the redundant lcd mutex introduced by commit 925ce689bb31 ("USB:
autoconvert trivial BKL users to private mutex") which replaced an
earlier BKL use.

The lock serialised calls to open() against other open() and a custom
ioctl() returning the bcdDevice (sic!), but neither is needed.

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/usblcd.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/drivers/usb/misc/usblcd.c b/drivers/usb/misc/usblcd.c
index b898650a5570..732eb1f81368 100644
--- a/drivers/usb/misc/usblcd.c
+++ b/drivers/usb/misc/usblcd.c
@@ -30,7 +30,6 @@
 #define IOCTL_GET_DRV_VERSION	2
 
 
-static DEFINE_MUTEX(lcd_mutex);
 static const struct usb_device_id id_table[] = {
 	{ .idVendor = 0x10D2, .match_flags = USB_DEVICE_ID_MATCH_VENDOR, },
 	{ },
@@ -81,12 +80,10 @@ static int lcd_open(struct inode *inode, struct file *file)
 	struct usb_interface *interface;
 	int subminor, r;
 
-	mutex_lock(&lcd_mutex);
 	subminor = iminor(inode);
 
 	interface = usb_find_interface(&lcd_driver, subminor);
 	if (!interface) {
-		mutex_unlock(&lcd_mutex);
 		printk(KERN_ERR "USBLCD: %s - error, can't find device for minor %d\n",
 		       __func__, subminor);
 		return -ENODEV;
@@ -101,13 +98,11 @@ static int lcd_open(struct inode *inode, struct file *file)
 	r = usb_autopm_get_interface(interface);
 	if (r < 0) {
 		kref_put(&dev->kref, lcd_delete);
-		mutex_unlock(&lcd_mutex);
 		return r;
 	}
 
 	/* save our object in the file's private structure */
 	file->private_data = dev;
-	mutex_unlock(&lcd_mutex);
 
 	return 0;
 }
@@ -176,14 +171,12 @@ static long lcd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 
 	switch (cmd) {
 	case IOCTL_GET_HARD_VERSION:
-		mutex_lock(&lcd_mutex);
 		bcdDevice = le16_to_cpu((dev->udev)->descriptor.bcdDevice);
 		sprintf(buf, "%1d%1d.%1d%1d",
 			(bcdDevice & 0xF000)>>12,
 			(bcdDevice & 0xF00)>>8,
 			(bcdDevice & 0xF0)>>4,
 			(bcdDevice & 0xF));
-		mutex_unlock(&lcd_mutex);
 		if (copy_to_user((void __user *)arg, buf, strlen(buf)) != 0)
 			return -EFAULT;
 		break;
-- 
2.23.0

[PATCH 4/4] USB: usblcd: use pr_err()

[Johan Hovold]

Replace the one remaining printk with pr_err().

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/usblcd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/misc/usblcd.c b/drivers/usb/misc/usblcd.c
index 732eb1f81368..61e9e987fe4a 100644
--- a/drivers/usb/misc/usblcd.c
+++ b/drivers/usb/misc/usblcd.c
@@ -84,7 +84,7 @@ static int lcd_open(struct inode *inode, struct file *file)
 
 	interface = usb_find_interface(&lcd_driver, subminor);
 	if (!interface) {
-		printk(KERN_ERR "USBLCD: %s - error, can't find device for minor %d\n",
+		pr_err("USBLCD: %s - error, can't find device for minor %d\n",
 		       __func__, subminor);
 		return -ENODEV;
 	}
-- 
2.23.0

Re: [PATCH 0/4] USB: usblcd: disconnect fix and locking clean ups

[Johan Hovold]

On Thu, Sep 26, 2019 at 11:12:24AM +0200, Johan Hovold wrote:
> This series fixes a failure to stop I/O on disconnect() in the usblcd
> driver. Turns out there was a lot of legacy cruft in this driver which
> could simply be removed.

My apologies for the double post.

Johan