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Subject: Re: [PATCH v2] kset: fix memory leak when kset_register() returns
 error
To:     Greg KH <gregkh@linuxfoundation.org>
CC:     <linux-kernel@vger.kernel.org>, <qemu-devel@nongnu.org>,
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        <alexander.deucher@amd.com>, <luben.tuikov@amd.com>,
        <richard@nod.at>, <liushixin2@huawei.com>,
        <yangyingliang@huawei.com>
References: <20221024121910.1169801-1-yangyingliang@huawei.com>
 <Y1aYuLmlXBRvMP1Z@kroah.com>
 <8281fc72-948a-162d-6e5f-a9fe29d8ee46@huawei.com>
 <Y1am4mjS+obAbUTJ@kroah.com>
From:   Yang Yingliang <yangyingliang@huawei.com>
Message-ID: <87e4e75b-a26e-6b4b-4799-c56c0b8891c0@huawei.com>
Date:   Mon, 24 Oct 2022 23:10:00 +0800
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On 2022/10/24 22:53, Greg KH wrote:
> On Mon, Oct 24, 2022 at 10:39:44PM +0800, Yang Yingliang wrote:
>> On 2022/10/24 21:52, Greg KH wrote:
>>> On Mon, Oct 24, 2022 at 08:19:10PM +0800, Yang Yingliang wrote:
>>>> Inject fault while loading module, kset_register() may fail.
>>>> If it fails, the name allocated by kobject_set_name() which
>>>> is called before kset_register() is leaked, because refcount
>>>> of kobject is hold in kset_init().
>>>>
>>>> As a kset may be embedded in a larger structure which needs
>>>> be freed in release() function or error path in callers, we
>>>> can not call kset_put() in kset_register(), or it will cause
>>>> double free, so just call kfree_const() to free the name and
>>>> set it to NULL.
>>>>
>>>> With this fix, the callers don't need to care about the name
>>>> freeing and call an extra kset_put() if kset_register() fails.
>>>>
>>>> Suggested-by: Luben Tuikov <luben.tuikov@amd.com>
>>>> Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
>>>> ---
>>>> v1 -> v2:
>>>>     Free name inside of kset_register() instead of calling kset_put()
>>>>     in drivers.
>>>> ---
>>>>    lib/kobject.c | 8 +++++++-
>>>>    1 file changed, 7 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/lib/kobject.c b/lib/kobject.c
>>>> index a0b2dbfcfa23..3409a89c81e5 100644
>>>> --- a/lib/kobject.c
>>>> +++ b/lib/kobject.c
>>>> @@ -834,6 +834,9 @@ EXPORT_SYMBOL_GPL(kobj_sysfs_ops);
>>>>    /**
>>>>     * kset_register() - Initialize and add a kset.
>>>>     * @k: kset.
>>>> + *
>>>> + * NOTE: On error, the kset.kobj.name allocated by() kobj_set_name()
>>>> + * which is called before kset_register() in caller need be freed.
>>> This comment doesn't make any sense anymore.  No caller needs to worry
>>> about this, right?
>> With this fix, the name is freed inside of kset_register(), it can not be
>> accessed,
> Agreed.
>
>> if it allocated dynamically, but callers don't know this if no comment here,
>> they may use it in error path (something like to print error message with
>> it),
>> so how about comment like this to tell callers not to use the name:
>>
>> NOTE: On error, the kset.kobj.name allocated by() kobj_set_name()
>> is freed, it can not be used any more.
> Sure, that's a better way to word it.
>
>>>>     */
>>>>    int kset_register(struct kset *k)
>>>>    {
>>>> @@ -844,8 +847,11 @@ int kset_register(struct kset *k)
>>>>    	kset_init(k);
>>>>    	err = kobject_add_internal(&k->kobj);
>>>> -	if (err)
>>>> +	if (err) {
>>>> +		kfree_const(k->kobj.name);
>>>> +		k->kobj.name = NULL;
>>> Why are you setting the name here to NULL?
>> I set it to NULL to avoid accessing bad pointer in callers,
>> if callers use it in error path, current callers won't use this
>> name pointer in error path, so we can remove this assignment?
> Ah, I didn't think about using it on error paths.  Ideally that would
> never happen, but that's good to set just to make it obvious.  How about
> adding a small comment here saying why you are setting it so we all
> remember it in 5 years when we look at the code again.
OK, I can add it in v3.

Thanks,
Yang
>
> thanks,
>
> greg k-h
> .