Re: [PATCH] comedi: Fix potential memory leak in comedi_init()

[shangxiaojing <shangxiaojing@huawei.com>]

On 2022/11/2 10:59, Greg KH wrote:
> On Wed, Nov 02, 2022 at 10:51:35AM +0800, shangxiaojing wrote:
>>
>>
>> On 2022/11/2 0:41, Ian Abbott wrote:
>>> On 01/11/2022 11:40, 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@huawei.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().
>>>
>>> Looking at it a bit more, cdev_init() calls kobject_init(), and
>>> kobject_init() requires a matching call to kobject_put().  Nothing is
>>> calling kobject_put() in this situation.  Calling cdev_del() will call
>>> kobject_put(), so is that the correct way to clean up after cdev_init()
>>> if the call to cdev_add() fails?
>>>
>>
>> Some cdev call cdev_del() when cdev_add() failed (like init_dvbdev()), and
>> at that time the cdev_unmap() in cdev_del() won't do anything due to the
>> failure of cdev_add(), which is calling the kobject_put() when cdev_add()
>> failed. But I'm not sure which one is better.
> 
> My point is that no one calling the cdev_add() call should have to do
> this type of "cleanup"  The cdev code should do it automatically, we
> don't want to have to audit every user of cdev_add() today, and in the
> future for forever, to ensure they got it right.
> 
> Let's fix it up in the cdev code itself please.  The kobject in a cdev
> is a very odd thing, it's not the "normal" type of kobject that you
> expect, so the cdev code should handle all of that on its own and that
> should not "leak" into any caller.
> 

ok, I'll fix in cdev code and send v2.

Thanks,
-- 
Shang XiaoJing