linux-kernel - Re: [PATCH v2] kset: fix memory leak when kset

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Date:   Mon, 24 Oct 2022 23:10:00 +0800
From:   Yang Yingliang <yangyingliang@...wei.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     <linux-kernel@...r.kernel.org>, <qemu-devel@...gnu.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>,
        <linux-erofs@...ts.ozlabs.org>, <ocfs2-devel@....oracle.com>,
        <linux-mtd@...ts.infradead.org>, <amd-gfx@...ts.freedesktop.org>,
        <rafael@...nel.org>, <somlo@....edu>, <mst@...hat.com>,
        <jaegeuk@...nel.org>, <chao@...nel.org>,
        <hsiangkao@...ux.alibaba.com>, <huangjianan@...o.com>,
        <mark@...heh.com>, <jlbec@...lplan.org>,
        <joseph.qi@...ux.alibaba.com>, <akpm@...ux-foundation.org>,
        <alexander.deucher@....com>, <luben.tuikov@....com>,
        <richard@....at>, <liushixin2@...wei.com>,
        <yangyingliang@...wei.com>
Subject: Re: [PATCH v2] kset: fix memory leak when kset_register() returns
 error


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@....com>
>>>> Signed-off-by: Yang Yingliang <yangyingliang@...wei.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
> .