netdev - Re: [PATCH 1/3] bridge: Fix error path for kobject_init_and

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Date:   Tue, 30 Apr 2019 10:23:13 -0500
From:   Tyler Hicks <tyhicks@...onical.com>
To:     "Tobin C. Harding" <tobin@...nel.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Ido Schimmel <idosch@...lanox.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Wang Hai <wanghai26@...wei.com>,
        YueHaibing <yuehaibing@...wei.com>,
        Amritha Nambiar <amritha.nambiar@...el.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] bridge: Fix error path for kobject_init_and_add()

On 2019-04-30 10:14:37, Tyler Hicks wrote:
> On 2019-04-30 10:28:15, Tobin C. Harding wrote:
> > Currently error return from kobject_init_and_add() is not followed by a
> > call to kobject_put().  This means there is a memory leak.
> > 
> > Add call to kobject_put() in error path of kobject_init_and_add().
> > 
> > Signed-off-by: Tobin C. Harding <tobin@...nel.org>
> > ---
> >  net/bridge/br_if.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/bridge/br_if.c b/net/bridge/br_if.c
> > index 41f0a696a65f..e5c8c9941c51 100644
> > --- a/net/bridge/br_if.c
> > +++ b/net/bridge/br_if.c
> > @@ -607,8 +607,10 @@ int br_add_if(struct net_bridge *br, struct net_device *dev,
> >  
> >  	err = kobject_init_and_add(&p->kobj, &brport_ktype, &(dev->dev.kobj),
> >  				   SYSFS_BRIDGE_PORT_ATTR);
> > -	if (err)
> > +	if (err) {
> > +		kobject_put(&p->kobj);
> >  		goto err1;
> > +	}
> 
> I think this is duplicating the code under the err2 label and doing so
> in a way that would introduce a double free.
> 
> If the refcount hits 0 in the kobject_put(), release_nbp() is called
> which calls kfree() on the p pointer. However, the p pointer is never
> set to NULL like what's done under the err2 label. Once we're back in
> br_add_if(), kfree() is called on the p pointer again just before
> returning.
> 
> I think it would be better if you just jumped to the err2 label instead
> of err1. err1 will no longer be used so, unfortunately, you'll have to
> refactor all the labels at the same time.

I noticed that the last paragraph is bad advice after looking at patch
#2 in this series. I guess you'd be better off calling kobject_put(),
setting p to NULL (the only thing that's missing), and jumping to err1
in this patch.

Tyler

> 
> Tyler
> 
> >  
> >  	err = br_sysfs_addif(p);
> >  	if (err)
> > -- 
> > 2.21.0
> >