[PATCH] char: misc: init list head only when needed

[PATCH] char: misc: init list head only when needed

[Jiazi Li]

If a module successfully registers a misc device.
Then, due to some bugs, use same address register misc device
again, init list head will corrupt misc_list, resulting in oops
when using misc_list.

In this scenario, do not init list head, if registration is
successful, init list head is also not required.

Signed-off-by: Jiazi Li <lijiazi@xiaomi.com>
---
 drivers/char/misc.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
index ca5141e..2451640 100644
--- a/drivers/char/misc.c
+++ b/drivers/char/misc.c
@@ -176,7 +176,6 @@ int misc_register(struct miscdevice *misc)
 	int err = 0;
 	bool is_dynamic = (misc->minor == MISC_DYNAMIC_MINOR);
 
-	INIT_LIST_HEAD(&misc->list);
 
 	mutex_lock(&misc_mtx);
 
@@ -185,7 +184,7 @@ int misc_register(struct miscdevice *misc)
 
 		if (i >= DYNAMIC_MINORS) {
 			err = -EBUSY;
-			goto out;
+			goto err2;
 		}
 		misc->minor = DYNAMIC_MINORS - i - 1;
 		set_bit(i, misc_minors);
@@ -195,7 +194,13 @@ int misc_register(struct miscdevice *misc)
 		list_for_each_entry(c, &misc_list, list) {
 			if (c->minor == misc->minor) {
 				err = -EBUSY;
-				goto out;
+				/*
+				 * if module use same address double register,
+				 * init list will corrupt misc_list
+				 */
+				if (c == misc)
+					goto err1;
+				goto err2;
 			}
 		}
 	}
@@ -214,7 +219,7 @@ int misc_register(struct miscdevice *misc)
 			misc->minor = MISC_DYNAMIC_MINOR;
 		}
 		err = PTR_ERR(misc->this_device);
-		goto out;
+		goto err2;
 	}
 
 	/*
@@ -222,7 +227,11 @@ int misc_register(struct miscdevice *misc)
 	 * earlier defaults
 	 */
 	list_add(&misc->list, &misc_list);
- out:
+	mutex_unlock(&misc_mtx);
+	return 0;
+ err2:
+	INIT_LIST_HEAD(&misc->list);
+ err1:
 	mutex_unlock(&misc_mtx);
 	return err;
 }
-- 
2.7.4

Re: [PATCH] char: misc: init list head only when needed

[Greg Kroah-Hartman]

On Mon, Aug 16, 2021 at 06:12:32PM +0800, Jiazi Li wrote:
> If a module successfully registers a misc device.
> Then, due to some bugs, use same address register misc device
> again, init list head will corrupt misc_list, resulting in oops
> when using misc_list.

Then fix those bugs that try to register the misc device more than once?

Does that happen in any in-kernel drivers or is this just a failure of
external drivers that are not submitted to the kernel tree?

> In this scenario, do not init list head, if registration is
> successful, init list head is also not required.
> 
> Signed-off-by: Jiazi Li <lijiazi@xiaomi.com>
> ---
>  drivers/char/misc.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/char/misc.c b/drivers/char/misc.c
> index ca5141e..2451640 100644
> --- a/drivers/char/misc.c
> +++ b/drivers/char/misc.c
> @@ -176,7 +176,6 @@ int misc_register(struct miscdevice *misc)
>  	int err = 0;
>  	bool is_dynamic = (misc->minor == MISC_DYNAMIC_MINOR);
>  
> -	INIT_LIST_HEAD(&misc->list);
>  
>  	mutex_lock(&misc_mtx);
>  
> @@ -185,7 +184,7 @@ int misc_register(struct miscdevice *misc)
>  
>  		if (i >= DYNAMIC_MINORS) {
>  			err = -EBUSY;
> -			goto out;
> +			goto err2;
>  		}
>  		misc->minor = DYNAMIC_MINORS - i - 1;
>  		set_bit(i, misc_minors);
> @@ -195,7 +194,13 @@ int misc_register(struct miscdevice *misc)
>  		list_for_each_entry(c, &misc_list, list) {
>  			if (c->minor == misc->minor) {
>  				err = -EBUSY;
> -				goto out;
> +				/*
> +				 * if module use same address double register,
> +				 * init list will corrupt misc_list

I do not understand the text here at all, sorry.

thanks,

greg k-h