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

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Date:   Tue, 1 Nov 2022 11:40:48 +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 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().

>>
> 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.

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