linux-kernel - Re: [PATCH 1/5] kprobes: Do not use local variable when creating debugfs file

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Date:   Wed, 9 Jun 2021 23:35:43 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Punit Agrawal <punitagrawal@...il.com>
Cc:     naveen.n.rao@...ux.ibm.com, anil.s.keshavamurthy@...el.com,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        guoren@...nel.org, linux-csky@...r.kernel.org
Subject: Re: [PATCH 1/5] kprobes: Do not use local variable when creating
 debugfs file

On Wed,  9 Jun 2021 19:50:15 +0900
Punit Agrawal <punitagrawal@...il.com> wrote:

> debugfs_create_file() takes a pointer argument that can be used during
> file operation callbacks (accessible via i_private in the inode
> structure). An obvious requirement is for the pointer to refer to
> valid memory when used.
> 
> When creating the debugfs file to dynamically enable / disable
> kprobes, a pointer to local variable is passed to
> debugfs_create_file(); which will go out of scope when the init
> function returns. The reason this hasn't triggered random memory
> corruption is because the pointer is not accessed during the debugfs
> file callbacks.
> 
> Fix the incorrect (and unnecessary) usage of local variable during
> debugfs_file_create() by passing NULL instead.
> 

Good catch! Since the enabled state is managed by the kprobes_all_disabled
global variable, it is not needed.

Fixes: bf8f6e5b3e51 ("Kprobes: The ON/OFF knob thru debugfs")
Acked-by: Masami Hiramatsu <mhiramat@...nel.org>

Thank you!

> Signed-off-by: Punit Agrawal <punitagrawal@...il.com>
> ---
>  kernel/kprobes.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 745f08fdd7a6..fdb1ea2e963b 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -2816,13 +2816,12 @@ static const struct file_operations fops_kp = {
>  static int __init debugfs_kprobe_init(void)
>  {
>  	struct dentry *dir;
> -	unsigned int value = 1;
>  
>  	dir = debugfs_create_dir("kprobes", NULL);
>  
>  	debugfs_create_file("list", 0400, dir, NULL, &kprobes_fops);
>  
> -	debugfs_create_file("enabled", 0600, dir, &value, &fops_kp);
> +	debugfs_create_file("enabled", 0600, dir, NULL, &fops_kp);
>  
>  	debugfs_create_file("blacklist", 0400, dir, NULL,
>  			    &kprobe_blacklist_fops);
> -- 
> 2.30.2
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>