Re: [PATCH v2] kset: fix memory leak when kset_register() returns error

[Greg KH <gregkh@linuxfoundation.org>]

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.

thanks,

greg k-h