Re: suspicious rcu_dereference_check in security/selinux/netnode.c

[Paul E. McKenney]

On Wed, Apr 20, 2011 at 03:29:59PM -0400, Paul Moore wrote:
> On Wednesday, April 20, 2011 2:42:04 PM Eric Paris wrote:
> > [added paul] EOM
> > 
> > On Wed, 2011-04-20 at 14:35 -0400, Dave Jones wrote:
> > > ===================================================
> > > 
> > >  [ INFO: suspicious rcu_dereference_check() usage. ]
> > >  ---------------------------------------------------
> > >  security/selinux/netnode.c:193 invoked rcu_dereference_check() without
> > >  protection!
> > >  
> > >  other info that might help us debug this:
> > >  
> > >  rcu_scheduler_active = 1, debug_locks = 0
> > >  
> > >  1 lock held by a.out/2018:
> > >   #0:  (sel_netnode_lock){+.....}, at: [<ffffffff81212ab7>]
> > >   sel_netnode_sid+0x9e/0x267
> > >  
> > >  stack backtrace:
> > >  Pid: 2018, comm: a.out Not tainted 2.6.39-rc4+ #3
> > >  
> > >  Call Trace:
> > >   [<ffffffff81084908>] lockdep_rcu_dereference+0xa8/0xb0
> > >   [<ffffffff81212c0d>] sel_netnode_sid+0x1f4/0x267
> > >   [<ffffffff81212a19>] ? sel_netnode_find+0xe3/0xe3
> > >   [<ffffffff8120d564>] selinux_socket_bind+0x1cf/0x26f
> > >   [<ffffffff81086c08>] ? lock_release+0x181/0x18e
> > >   [<ffffffff81100db8>] ? might_fault+0xa5/0xac
> > >   [<ffffffff81100d6f>] ? might_fault+0x5c/0xac
> > >   [<ffffffff812073f1>] security_socket_bind+0x16/0x18
> > >   [<ffffffff813ee0e9>] sys_bind+0x73/0xcf
> > >   [<ffffffff814c5d7a>] ? sysret_check+0x2e/0x69
> > >   [<ffffffff810870cf>] ? trace_hardirqs_on_caller+0x10b/0x12f
> > >   [<ffffffff810a9efb>] ? audit_syscall_entry+0x11c/0x148
> > >   [<ffffffff81255e2e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> > >   [<ffffffff814c5d42>] system_call_fastpath+0x16/0x1b
> > > 
> > > something like this perhaps ?
> > > 
> > > 	Dave
> > > 
> > > diff --git a/security/selinux/netnode.c b/security/selinux/netnode.c
> > > index 65ebfe9..d0c38ba 100644
> > > --- a/security/selinux/netnode.c
> > > +++ b/security/selinux/netnode.c
> > > @@ -188,9 +188,11 @@ static void sel_netnode_insert(struct sel_netnode
> > > *node)
> > > 
> > >  	list_add_rcu(&node->list, &sel_netnode_hash[idx].list);
> > >  	if (sel_netnode_hash[idx].size == SEL_NETNODE_HASH_BKT_LIMIT) {
> > >  	
> > >  		struct sel_netnode *tail;
> > > 
> > > +		rcu_read_lock();
> > > 
> > >  		tail = list_entry(
> > >  		
> > >  			rcu_dereference(sel_netnode_hash[idx].list.prev),
> > >  			struct sel_netnode, list);
> > > 
> > > +		rcu_read_unlock();
> > > 
> > >  		list_del_rcu(&tail->list);
> > >  		call_rcu(&tail->rcu, sel_netnode_free);
> > >  	
> > >  	} else
> 
> [Ooops, forgot to hit reply-all on the first attempt]
> 
> Hmm, I think the correct fix might be to just remove the rcu_dereference() 
> call since this is protected by a spin lock (see sel_netnode_sid_slow()).  I 
> may be wrong, but I thought rcu locks/derefs were not needed when a spin lock 
> was held, yes?
> 
> Regardless of the fix, the same thing should probably be done to the 
> sel_netport_* versions of these functions.

The lock is sel_netnode_lock, correct?  Then the best approach is as
follows:

 		tail = list_entry(
 			rcu_dereference_protected(sel_netnode_hash[idx].list.prev,
						  lockdep_is_held(&sel_netnode_lock)),
 			struct sel_netnode, list);

Give or take long lines, anyway...  :-(

This way, if someone mistakenly calls this function without holding
the lock, CONFIG_PROVE_RCU will know to complain.

And Paul Moore is quite correct when he says that rcu_read_lock() is
not needed in this case.

							Thanx, Paul
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