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

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Date:   Fri, 11 Jun 2021 08:31:44 +0900
From:   Punit Agrawal <punitagrawal@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
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

Hi Masami,

Masami Hiramatsu <mhiramat@...nel.org> writes:

> 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>

Thanks a lot for reviewing the patches.

I am assuming the tags can be picked up when applying. Let me know if I
need to resend.

Thanks,
Punit

>
> 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
>>