linux-kernel - Re: [PATCH] comedi: Fix potential memory leak in comedi

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Date:   Wed, 2 Nov 2022 10:41:56 +0800
From:   shangxiaojing <shangxiaojing@...wei.com>
To:     Ian Abbott <abbotti@....co.uk>,
        Greg KH <gregkh@...uxfoundation.org>
CC:     <hsweeten@...ionengravers.com>, <zhangxuezhi1@...lpad.com>,
        <fmhess@...rs.sourceforge.net>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] comedi: Fix potential memory leak in comedi_init()



On 2022/11/1 23:59, Ian Abbott wrote:
> On 01/11/2022 11:57, shangxiaojing wrote:
>>
>>
>> On 2022/11/1 19:40, Ian Abbott wrote:
>>> On 01/11/2022 06:16, shangxiaojing wrote:
>>>>
>>>>
>>>> On 2022/11/1 12:45, Greg KH wrote:
>>>>> On Tue, Nov 01, 2022 at 11:21:25AM +0800, Shang XiaoJing wrote:
>>>>>> comedi_init() will goto out_unregister_chrdev_region if cdev_add()
>>>>>> failed, which won't free the resource alloced in kobject_set_name().
>>>>>> Call kfree_const() to free the leaked name before goto
>>>>>> out_unregister_chrdev_region.
>>>>>>
>>>>>> unreferenced object 0xffff8881000fa8c0 (size 8):
>>>>>>    comm "modprobe", pid 239, jiffies 4294905173 (age 51.308s)
>>>>>>    hex dump (first 8 bytes):
>>>>>>      63 6f 6d 65 64 69 00 ff                          comedi..
>>>>>>    backtrace:
>>>>>>      [<000000005f9878f7>] __kmalloc_node_track_caller+0x4c/0x1c0
>>>>>>      [<000000000fd70302>] kstrdup+0x3f/0x70
>>>>>>      [<000000009428bc33>] kstrdup_const+0x46/0x60
>>>>>>      [<00000000ed50d9de>] kvasprintf_const+0xdb/0xf0
>>>>>>      [<00000000b2766964>] kobject_set_name_vargs+0x3c/0xe0
>>>>>>      [<00000000f2424ef7>] kobject_set_name+0x62/0x90
>>>>>>      [<000000005d5a125b>] 0xffffffffa0013098
>>>>>>      [<00000000f331e663>] do_one_initcall+0x7a/0x380
>>>>>>      [<00000000aa7bac96>] do_init_module+0x5c/0x230
>>>>>>      [<000000005fd72335>] load_module+0x227d/0x2420
>>>>>>      [<00000000ad550cf1>] __do_sys_finit_module+0xd5/0x140
>>>>>>      [<00000000069a60c5>] do_syscall_64+0x3f/0x90
>>>>>>      [<00000000c5e0d521>] entry_SYSCALL_64_after_hwframe+0x63/0xcd
>>>>>>
>>>>>> Fixes: ed9eccbe8970 ("Staging: add comedi core")
>>>>>> Signed-off-by: Shang XiaoJing <shangxiaojing@...wei.com>
>>>>>> ---
>>>>>>   drivers/comedi/comedi_fops.c | 5 ++++-
>>>>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/comedi/comedi_fops.c 
>>>>>> b/drivers/comedi/comedi_fops.c
>>>>>> index e2114bcf815a..2c508c2cf6f6 100644
>>>>>> --- a/drivers/comedi/comedi_fops.c
>>>>>> +++ b/drivers/comedi/comedi_fops.c
>>>>>> @@ -3379,8 +3379,11 @@ static int __init comedi_init(void)
>>>>>>       retval = cdev_add(&comedi_cdev, MKDEV(COMEDI_MAJOR, 0),
>>>>>>                 COMEDI_NUM_MINORS);
>>>>>> -    if (retval)
>>>>>> +    if (retval) {
>>>>>> +        kfree_const(comedi_cdev.kobj.name);
>>>>>> +        comedi_cdev.kobj.name = NULL;
>>>>>>           goto out_unregister_chrdev_region;
>>>>>> +    }
>>>>>
>>>>> A driver should never have to poke around in the internals of a cdev
>>>>> object like this.  Please fix the cdev core to not need this if
>>>>> cdev_add() fails.
>>>
>>> Or at least there should be a function that can be called to undo the 
>>> allocations of kobject_set_name().
>>
>> It's ok to add a pair function, but maybe only one place where used 
>> cdev_add() need free name. Your mean all "kfree_const(name);" should 
>> be replaced to the new function?
>>
>>>
>>>>>
>>>> Hi,
>>>>
>>>> I had checked all 67 places that used cdev_add(), and only the 
>>>> following 5 functions set the name before the cdev_add():
>>>>
>>>> 1. comedi_init(), will memleak and will be fixed by this patch.
>>>> 2. hfi1_cdev_init(), won't memleak.
>>>> 3. qib_cdev_init(), won't memleak.
>>>> 4. uio_major_init(), won't memleak.
>>>> 5. __register_chrdev(), won't memleak.
>>>>
>>>> As the result, I just fix the code in comedi_init(). Should I still 
>>>> fix the cdev core code, which is free the name in cdev_add() fail path?
>>>
>>> I'm stuck trying to work out why hfi1_cdev_init() doesn't have the 
>>> same problem when cdev_add() returns an error.
>>>
>>
>> Only user_add() calls the hfi1_cdev_init(), and will call 
>> user_remove() if hfi1_cdev_init() failed. In user_remove(), 
>> hfi1_cdev_cleanup() will call cdev_del().
> 
> No it won't.  hfi1_cdev_cleanup() only calls cdev_del() if device is 
> non-null, but device will be null if the call to cdev_add() failed in 
> hfi1_cdev_init().
> 

Right, device won't be inited, so there has the same potential memleak.

Thanks,
-- 
Shang XiaoJing