linux-kernel - Re: Re: [PATCH net V4 1/2] ax25: Fix refcount leaks caused by ax25_cb

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Date:   Tue, 15 Mar 2022 17:19:05 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     周多明 <duoming@....edu.cn>
Cc:     linux-hams@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kuba@...nel.org, davem@...emloft.net,
        ralf@...ux-mips.org, jreuter@...na.de, thomas@...erried.de
Subject: Re: Re: [PATCH net V4 1/2] ax25: Fix refcount leaks caused by
 ax25_cb_del()

On Tue, Mar 15, 2022 at 10:11:10PM +0800, 周多明 wrote:
> Hello,
> 
> On Tue, 15 Mar 2022 13:26:57 +0300, Dan Carpenter wrote:
> > I'm happy that this is simpler.  I'm not super happy about the
> > if (sk->sk_wq) check.  That seems like a fragile side-effect condition
> > instead of something deliberate.  But I don't know networking so maybe
> > this is something which we can rely on.
> 
> The variable sk->sk_wq is the address of waiting queue of sock, it is initialized to the 
> address of sock->wq through the following path:
> sock_create->__sock_create->ax25_create()->sock_init_data()->RCU_INIT_POINTER(sk->sk_wq, &sock->wq).
> Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or the address of
> sock->wq is not null.
> What`s more, sk->sk_wq is set to null only in sock_orphan().
> 
> Another solution:
> We could also use sk->sk_socket to check. We set sk->sk_socket to sock in the following path:
> sock_create()->__sock_create()->ax25_create()->sock_init_data()->sk_set_socket(sk, sock).
> Because we have used sock_alloc() to allocate the socket in __sock_create(), sock or sk->sk_socket
> is not null.
> What`s more, sk->sk_socket is set to null only in sock_orphan().
> 
> I will change the if (sk->sk_wq) check to if(sk->sk_socket) check, because I think it is 
> easier to understand.
> 
> > When you sent the earlier patch then I asked if the devices in
> > ax25_kill_by_device() were always bound and if we could just use a local
> > variable instead of something tied to the ax25_dev struct.  I still
> > wonder about that.  In other words, could we just do this?
> > 
> > diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> > index 6bd097180772..4af9d9a939c6 100644
> > --- a/net/ax25/af_ax25.c
> > +++ b/net/ax25/af_ax25.c
> > @@ -78,6 +78,7 @@ static void ax25_kill_by_device(struct net_device *dev)
> >  	ax25_dev *ax25_dev;
> >  	ax25_cb *s;
> >  	struct sock *sk;
> > +	bool found = false;
> >  
> >  	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
> >  		return;
> > @@ -86,6 +87,7 @@ static void ax25_kill_by_device(struct net_device *dev)
> >  again:
> >  	ax25_for_each(s, &ax25_list) {
> >  		if (s->ax25_dev == ax25_dev) {
> > +			found = true;
> >  			sk = s->sk;
> >  			if (!sk) {
> >  				spin_unlock_bh(&ax25_list_lock);
> > @@ -115,6 +117,11 @@ static void ax25_kill_by_device(struct net_device *dev)
> >  		}
> >  	}
> >  	spin_unlock_bh(&ax25_list_lock);
> > +
> > +	if (!found) {
> > +		dev_put_track(ax25_dev->dev, &ax25_dev->dev_tracker);
> > +		ax25_dev_put(ax25_dev);
> > +	}
> >  }
> 
> If we just use ax25_dev_device_up() to bring device up without using ax25_bind(),
> the "found" flag could be false when we enter ax25_kill_by_device() and the refcounts 
> underflow will happen. So we should use two additional variables.

That answers my question.  Thank you.

> 
> If we use additional variables to fix the bug, I think there is a problem.

So the v3 patch was buggy?

Why was this not explained under the --- cut off line?

regards,
dan carpenter