core-1					core-2
-------------------------------------------------------------
uio_unregister_device			uio_open
					idev = idr_find()
device_unregister(&idev->dev)
put_device(&idev->dev)
uio_device_release
					get_device(&idev->dev)
kfree(idev)
uio_free_minor(minor)
					uio_release
					put_device(&idev->dev)
					kfree(idev)
-------------------------------------------------------------

In the core-1 uio_unregister_device(), the device_unregister will kfree idev
when the idev->dev kobject ref is one. But after core-1 device_unregister,
put_device and before doing kfree, the core-2 may get_device. Then:
1. After core-1 kfree idev, the core-2 will do use-after-free for idev.
2. When core-2 do uio_release and put_device, the idev will be double freed.

To address this issue, we can get idev atomic & inc idev reference with minor_lock.

Signed-off-by: Guanghui Feng <guanghuifeng@linux.alibaba.com>
---
 drivers/uio/uio.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
index 62082d6..2d572f6 100644
--- a/drivers/uio/uio.c
+++ b/drivers/uio/uio.c
@@ -466,13 +466,13 @@ static int uio_open(struct inode *inode, struct file *filep)
 
 	mutex_lock(&minor_lock);
 	idev = idr_find(&uio_idr, iminor(inode));
-	mutex_unlock(&minor_lock);
 	if (!idev) {
 		ret = -ENODEV;
+		mutex_unlock(&minor_lock);
 		goto out;
 	}
-
 	get_device(&idev->dev);
+	mutex_unlock(&minor_lock);
 
 	if (!try_module_get(idev->owner)) {
 		ret = -ENODEV;
@@ -1064,9 +1064,8 @@ void uio_unregister_device(struct uio_info *info)
 	wake_up_interruptible(&idev->wait);
 	kill_fasync(&idev->async_queue, SIGIO, POLL_HUP);
 
-	device_unregister(&idev->dev);
-
 	uio_free_minor(minor);
+	device_unregister(&idev->dev);
 
 	return;
 }
-- 
1.8.3.1

On Thu, Dec 21, 2023 at 12:26:48PM +0800, Guanghui Feng wrote:
> core-1					core-2
> -------------------------------------------------------------
> uio_unregister_device			uio_open
> 					idev = idr_find()
> device_unregister(&idev->dev)
> put_device(&idev->dev)
> uio_device_release
> 					get_device(&idev->dev)
> kfree(idev)
> uio_free_minor(minor)
> 					uio_release
> 					put_device(&idev->dev)
> 					kfree(idev)
> -------------------------------------------------------------
> 
> In the core-1 uio_unregister_device(), the device_unregister will kfree idev
> when the idev->dev kobject ref is one. But after core-1 device_unregister,
> put_device and before doing kfree, the core-2 may get_device. Then:
> 1. After core-1 kfree idev, the core-2 will do use-after-free for idev.
> 2. When core-2 do uio_release and put_device, the idev will be double freed.
> 
> To address this issue, we can get idev atomic & inc idev reference with minor_lock.

Nit, can you wrap your lines at 72 columns?

Anyway, nice fix, how did you find this?

> Signed-off-by: Guanghui Feng <guanghuifeng@linux.alibaba.com>

What commit id does this fix?

thanks,

greg k-h

core-1				core-2
-------------------------------------------------------
uio_unregister_device		uio_open
				idev = idr_find()
device_unregister(&idev->dev)
put_device(&idev->dev)
uio_device_release
				get_device(&idev->dev)
kfree(idev)
uio_free_minor(minor)
				uio_release
				put_device(&idev->dev)
				kfree(idev)
-------------------------------------------------------

In the core-1 uio_unregister_device(), the device_unregister will kfree
idev when the idev->dev kobject ref is 1. But after core-1
device_unregister, put_device and before doing kfree, the core-2 may
get_device. Then:
1. After core-1 kfree idev, the core-2 will do use-after-free for idev.
2. When core-2 do uio_release and put_device, the idev will be double
   freed.

To address this issue, we can get idev atomic & inc idev reference with
minor_lock.

Fixes: 57c5f4df0a5a ("uio: fix crash after the device is unregistered")
Signed-off-by: Guanghui Feng <guanghuifeng@linux.alibaba.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 drivers/uio/uio.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
index 62082d6..2d572f6 100644
--- a/drivers/uio/uio.c
+++ b/drivers/uio/uio.c
@@ -466,13 +466,13 @@ static int uio_open(struct inode *inode, struct file *filep)
 
 	mutex_lock(&minor_lock);
 	idev = idr_find(&uio_idr, iminor(inode));
-	mutex_unlock(&minor_lock);
 	if (!idev) {
 		ret = -ENODEV;
+		mutex_unlock(&minor_lock);
 		goto out;
 	}
-
 	get_device(&idev->dev);
+	mutex_unlock(&minor_lock);
 
 	if (!try_module_get(idev->owner)) {
 		ret = -ENODEV;
@@ -1064,9 +1064,8 @@ void uio_unregister_device(struct uio_info *info)
 	wake_up_interruptible(&idev->wait);
 	kill_fasync(&idev->async_queue, SIGIO, POLL_HUP);
 
-	device_unregister(&idev->dev);
-
 	uio_free_minor(minor);
+	device_unregister(&idev->dev);
 
 	return;
 }
-- 
1.8.3.1