Re: [PATCH] net: add a synchronize_net() innetdev_rx_handler_unregister()

[Paul E. McKenney]

On Fri, Mar 29, 2013 at 06:01:22AM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
> 
> On Fri, 2013-03-29 at 10:48 +0100, Jiri Pirko wrote:
> 
> > Hmm. I think that this might be issue introduced by:
> > commit a9b3cd7f323b2e57593e7215362a7b02fc933e3a
> > Author: Stephen Hemminger <shemminger@xxxxxxxxxx>
> > Date:   Mon Aug 1 16:19:00 2011 +0000
> > 
> >     rcu: convert uses of rcu_assign_pointer(x, NULL) to RCU_INIT_POINTER
> > 
> > 
> > Because, if rcu_dereference(dev->rx_handler) is null,
> > rcu_dereference(dev->rx_handler_data) is never done. Therefore I believe
> > we are hitting following scenario:
> > 
> > 
> >    CPU0				CPU1
> >    ----				----
> >   			    dev->rx_handler_data = NULL
> >  rcu_read_lock()
> >  			    dev->rx_handler = NULL
> > 
> > 
> > CPU0 will see rx_handler set and yet, rx_handler_data nulled. Write
> > barrier in rcu_assign_pointer() might prevent this reorder from happening.
> > Therefore I suggest:
> > 
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 0caa38e..c16b829 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -3332,8 +3332,8 @@ void netdev_rx_handler_unregister(struct net_device *dev)
> >  {
> >  
> >  	ASSERT_RTNL();
> > -	RCU_INIT_POINTER(dev->rx_handler, NULL);
> > -	RCU_INIT_POINTER(dev->rx_handler_data, NULL);
> > +	rcu_assign_pointer(dev->rx_handler, NULL);
> > +	rcu_assign_pointer(dev->rx_handler_data, NULL);
> >  }
> >  EXPORT_SYMBOL_GPL(netdev_rx_handler_unregister);
> >  
> > 
> 
> Nope this changes nothing at all.
> 
> However, we can fix the bug in a different way, if we want to avoid a
> test in fast path.
> 
> With following patch, we can make sure that a reader seeing a non NULL
> rx_handler has a guarantee to see a non NULL rx_handler_data
> 
> Thanks
> 
> [PATCH] net: add a synchronize_net() in netdev_rx_handler_unregister()
> 
> commit 35d48903e97819 (bonding: fix rx_handler locking) added a race
> in bonding driver, reported by Steven Rostedt who did a very good
> diagnosis :
> 
> <quoting Steven>
> 
> I'm currently debugging a crash in an old 3.0-rt kernel that one of our
> customers is seeing. The bug happens with a stress test that loads and
> unloads the bonding module in a loop (I don't know all the details as
> I'm not the one that is directly interacting with the customer). But the
> bug looks to be something that may still be present and possibly present
> in mainline too. It will just be much harder to trigger it in mainline.
> 
> In -rt, interrupts are threads, and can schedule in and out just like
> any other thread. Note, mainline now supports interrupt threads so this
> may be easily reproducible in mainline as well. I don't have the ability
> to tell the customer to try mainline or other kernels, so my hands are
> somewhat tied to what I can do.
> 
> But according to a core dump, I tracked down that the eth irq thread
> crashed in bond_handle_frame() here:
> 
>         slave = bond_slave_get_rcu(skb->dev);
>         bond = slave->bond; <--- BUG
> 
> 
> the slave returned was NULL and accessing slave->bond caused a NULL
> pointer dereference.
> 
> Looking at the code that unregisters the handler:
> 
> void netdev_rx_handler_unregister(struct net_device *dev)
> {
> 
>         ASSERT_RTNL();
>         RCU_INIT_POINTER(dev->rx_handler, NULL);
>         RCU_INIT_POINTER(dev->rx_handler_data, NULL);
> }
> 
> Which is basically:
>         dev->rx_handler = NULL;
>         dev->rx_handler_data = NULL;
> 
> And looking at __netif_receive_skb() we have:
> 
>         rx_handler = rcu_dereference(skb->dev->rx_handler);
>         if (rx_handler) {
>                 if (pt_prev) {
>                         ret = deliver_skb(skb, pt_prev, orig_dev);
>                         pt_prev = NULL;
>                 }
>                 switch (rx_handler(&skb)) {
> 
> My question to all of you is, what stops this interrupt from happening
> while the bonding module is unloading?  What happens if the interrupt
> triggers and we have this:
> 
> 
>         CPU0                    CPU1
>         ----                    ----
>   rx_handler = skb->dev->rx_handler
> 
>                         netdev_rx_handler_unregister() {
>                            dev->rx_handler = NULL;
>                            dev->rx_handler_data = NULL;
> 
>   rx_handler()
>    bond_handle_frame() {
>     slave = skb->dev->rx_handler;
>     bond = slave->bond; <-- NULL pointer dereference!!!
> 
> 
> What protection am I missing in the bond release handler that would
> prevent the above from happening?
> 
> </quoting Steven>
> 
> We can fix bug this in two ways. First is adding a test in
> bond_handle_frame() and others to check if rx_handler_data is NULL.
> 
> A second way is adding a synchronize_net() in
> netdev_rx_handler_unregister() to make sure that a rcu protected reader
> has the guarantee to see a non NULL rx_handler_data.
> 
> The second way is better as it avoids an extra test in fast path.
> 
> Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Jiri Pirko <jpirko@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

With kudos to Steven Rostedt for his analogy between RCU and
Schrödinger's cat.  ;-)

> ---
>  net/core/dev.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index b13e5c7..56932a4 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3314,6 +3314,7 @@ int netdev_rx_handler_register(struct net_device *dev,
>  	if (dev->rx_handler)
>  		return -EBUSY;
> 
> +	/* Note: rx_handler_data must be set before rx_handler */
>  	rcu_assign_pointer(dev->rx_handler_data, rx_handler_data);
>  	rcu_assign_pointer(dev->rx_handler, rx_handler);
> 
> @@ -3334,6 +3335,11 @@ void netdev_rx_handler_unregister(struct net_device *dev)
> 
>  	ASSERT_RTNL();
>  	RCU_INIT_POINTER(dev->rx_handler, NULL);
> +	/* a reader seeing a non NULL rx_handler in a rcu_read_lock()
> +	 * section has a guarantee to see a non NULL rx_handler_data
> +	 * as well.
> +	 */
> +	synchronize_net();
>  	RCU_INIT_POINTER(dev->rx_handler_data, NULL);
>  }
>  EXPORT_SYMBOL_GPL(netdev_rx_handler_unregister);
> 
> 
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