linux-kernel - Re: suspicious RCU usage in security/selinux/netnode.c

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Date:	Tue, 15 May 2012 11:12:27 -0400
From:	Paul Moore <paul@...l-moore.com>
To:	Eric Paris <eparis@...isplace.org>
Cc:	paulmck@...ux.vnet.ibm.com, Dave Jones <davej@...hat.com>,
	sds@...ho.nsa.gov, Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: suspicious RCU usage in security/selinux/netnode.c

On Tuesday, May 15, 2012 10:52:07 AM Eric Paris wrote:
> On Tue, May 15, 2012 at 10:46 AM, Paul E. McKenney
> 
> <paulmck@...ux.vnet.ibm.com> wrote:
> > On Tue, May 15, 2012 at 10:24:23AM -0400, Eric Paris wrote:
> >> On Tue, May 15, 2012 at 1:16 AM, Paul E. McKenney
> >> 
> >> <paulmck@...ux.vnet.ibm.com> wrote:
> >> > On Tue, May 15, 2012 at 12:41:45AM -0400, Dave Jones wrote:
> >> >> I just triggered this on Linus' current tree.
> >> > 
> >> > This is a bare:
> >> > 
> >> >        rcu_dereference(sel_netnode_hash[idx].list.prev)
> >> > 
> >> > which needs to be in an RCU read-side critical section.  Alternatively,
> >> > the above should instead be something like:
> >> > 
> >> >        rcu_dereference_check(sel_netnode_hash[idx].list.prev,
> >> >                              lockdep_is_held(&sel_netnode_lock));
> >> 
> >> Right, but that 'bare' dereference comes from
> >> list_for_each_entry_rcu(), [from sel_netnode_sid_slow()] which I don't
> >> see how to easily annotate with the lock.  Nor do I think it's within
> >> my brain power (or my willingness to maintain such in the future) to
> >> want to open code that logic.
> > 
> > You lost me on this one.  The lockdep splat called out the
> > rcu_dereference() above, not a list_for_each_entry_rcu().  Besides which,
> > the list_for_each_entry_rcu() does not do the checking -- at the time,
> > I was not willing to explode the API that much.
> 
> Ohhhh, ok.  I assumed we needed to annotate list_for_each_entry_rcu()
> under the spinlock as well as the bare dereference in the insert code.
>  Ok, should be very easy to fix, although the list running code is
> still going to be un-annotated in any way.  Thanks

Sorry, email filters went awry and I lost this thread until Eric pointed it 
out to me ...

Despite a common first name, the other Paul is the RCU expert, no I 
unfortunately.  Can someone explain the difference between 
rcu_dereference_check() and rcu_dereference_protected()?  We use 
rcu_dereference_protected() for a very similar reason in 
selinux/netport.c:sel_netport_insert() and it seems like a better choice ... ?

I'll throw a patch together but wanted to clear this up first.

-- 
paul moore
www.paul-moore.com

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