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

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Message-ID: <83f1aa93-bbd9-925e-a1d4-26729262a008@mev.co.uk>
Date:   Tue, 1 Nov 2022 15:59:50 +0000
From:   Ian Abbott <abbotti@....co.uk>
To:     shangxiaojing <shangxiaojing@...wei.com>,
        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 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().

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