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

From: Paul E. McKenney
Date: Tue May 15 2012 - 10:47:24 EST

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@xxxxxxxxxxxxxxxxxx> 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.

> Should we just take the rcu_read_lock() where we take the spinlock?
> Is that a perf hit and figuring out how to do the annotation correctly
> is the better idea?

If the spinlock is protecting the data, then just add the spinlock
to the rcu_dereference_check() as shown above.

Thanx, Paul

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