netdev - Re: [PATCH] bridge: fix bridge netlink RCU usage

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Date:	Tue, 03 Mar 2015 14:43:33 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, Roopa Prabhu <roopa@...ulusnetworks.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH] bridge: fix bridge netlink RCU usage

On Tue, 2015-03-03 at 05:40 -0800, Eric Dumazet wrote:
> On Tue, 2015-03-03 at 14:29 +0100, Johannes Berg wrote:
> > From: Johannes Berg <johannes.berg@...el.com>
> > 
> > When the STP timer fires, it can call br_ifinfo_notify(),
> > which in turn ends up in the new br_get_link_af_size().
> > This function is annotated to be using RTNL locking, which
> > clearly isn't the case here, and thus lockdep warns:
> > 
> >   ===============================
> >   [ INFO: suspicious RCU usage. ]
> >   3.19.0+ #569 Not tainted
> >   -------------------------------
> >   net/bridge/br_private.h:204 suspicious rcu_dereference_protected() usage!
> > 
> > Fix this by doing RCU locking here.
> > 
> > Fixes: b7853d73e39b ("bridge: add vlan info to bridge setlink and dellink notification messages")
> > Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> > ---
> >  net/bridge/br_netlink.c | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/bridge/br_netlink.c b/net/bridge/br_netlink.c
> > index 17e0177467f5..c63ac0d13add 100644
> > --- a/net/bridge/br_netlink.c
> > +++ b/net/bridge/br_netlink.c
> > @@ -26,12 +26,15 @@ static size_t br_get_link_af_size(const struct net_device *dev)
> >  {
> >  	struct net_port_vlans *pv;
> >  
> > -	if (br_port_exists(dev))
> > -		pv = nbp_get_vlan_info(br_port_get_rtnl(dev));
> > -	else if (dev->priv_flags & IFF_EBRIDGE)
> > +	if (br_port_exists(dev)) {
> > +		rcu_read_lock();
> > +		pv = nbp_get_vlan_info(br_port_get_rcu(dev));
> > +		rcu_read_unlock();
> 
> right after this rcu_read_unlock(), you no longer are allowed to deref
> pv

Right, that's what I feared, hence my other email :)

But if I reorder it to later it would be OK?

johannes

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