Subject: Re: [PATCH v4] net: rose: fix null-ptr-deref caused by rose_kill_by_neigh

From: Jakub Kicinski <[email-masked]>
Date: Sat, 2 Jul 2022 12:01:08 -0700

On Sat, 2 Jul 2022 15:23:57 +0800 (GMT+08:00) [email-masked] wrote:
> > On Wed, 29 Jun 2022 18:49:41 +0800 Duoming Zhou wrote:  
> > > When the link layer connection is broken, the rose->neighbour is
> > > set to null. But rose->neighbour could be used by rose_connection()
> > > and rose_release() later, because there is no synchronization among
> > > them. As a result, the null-ptr-deref bugs will happen.
> > > 
> > > One of the null-ptr-deref bugs is shown below:
> > > 
> > >     (thread 1)                  |        (thread 2)
> > >                                 |  rose_connect
> > > rose_kill_by_neigh              |    lock_sock(sk)
> > >   spin_lock_bh(&rose_list_lock) |    if (!rose->neighbour)
> > >   rose->neighbour = NULL;//(1)  |
> > >                                 |    rose->neighbour->use++;//(2)  
> >   
> > >  		if (rose->neighbour == neigh) {  
> > 
> > Why is it okay to perform this comparison without the socket lock,
> > if we need a socket lock to clear it? Looks like rose_kill_by_neigh()
> > is not guaranteed to clear all the uses of a neighbor.  
> 
> I am sorry, the comparision should also be protected with socket lock.
> The rose_kill_by_neigh() only clear the neighbor that is passed as
> parameter of rose_kill_by_neigh(). 

Don't think that's possible, you'd have to drop the neigh lock every
time.

> > > +			sock_hold(s);
> > > +			spin_unlock_bh(&rose_list_lock);
> > > +			lock_sock(s);
> > >  			rose_disconnect(s, ENETUNREACH, ROSE_OUT_OF_ORDER, 0);
> > >  			rose->neighbour->use--;  
> > 
> > What protects the use counter?  
> 
> The use coounter is protected by socket lock.

Which one, the neigh object can be shared by multiple sockets, no?