Re: [patch net/stable v2] br: fix use of ->rx_handler_data in code executed on non-rx_handler path

Stephen Hemminger
2013-12-07T17:42:39Z

On Sat, 7 Dec 2013 09:51:05 +0100
Jiri Pirko <jiri@resnulli.us> wrote:

> Fri, Dec 06, 2013 at 10:10:28PM CET, stephen@networkplumber.org wrote:
> >On Fri, 06 Dec 2013 15:43:21 -0500 (EST)
> >David Miller <davem@davemloft.net> wrote:
> >
> >> From: Jiri Pirko <jiri@resnulli.us>
> >> Date: Thu, 5 Dec 2013 16:27:37 +0100
> >>
> >> > br_stp_rcv() is reached by non-rx_handler path. That means there is no
> >> > guarantee that dev is bridge port and therefore simple NULL check of
> >> > ->rx_handler_data is not enough. There is need to check if dev is really
> >> > bridge port and since only rcu read lock is held here, do it by checking
> >> > ->rx_handler pointer.
> >> >
> >> > Note that synchronize_net() in netdev_rx_handler_unregister() ensures
> >> > this approach as valid.
> >> >
> >> > Introduced originally by:
> >> > commit f350a0a87374418635689471606454abc7beaa3a
> >> > "bridge: use rx_handler_data pointer to store net_bridge_port pointer"
> >> >
> >> > Fixed but not in the best way by:
> >> > commit b5ed54e94d324f17c97852296d61a143f01b227a
> >> > "bridge: fix RCU races with bridge port"
> >> >
> >> > Reintroduced by:
> >> > commit 716ec052d2280d511e10e90ad54a86f5b5d4dcc2
> >> > "bridge: fix NULL pointer deref of br_port_get_rcu"
> >> >
> >> > Please apply to stable trees as well. Thanks.
> >> >
> >> > RH bugzilla reference: https://bugzilla.redhat.com/show_bug.cgi?id=1025770
> >> >
> >> > Reported-by: Laine Stump <laine@redhat.com>
> >> > Debugged-by: Michael S. Tsirkin <mst@redhat.com>
> >> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> >> > Signed-off-by: Jiri Pirko <jiri@resnulli.us>
> >> > ---
> >> > v1->v2: moved br_port_get_check_rcu definition below br_handle_frame definition
> >>
> >> Applied and queued up for -stable, thanks Jiri.
> >
> >How come you ignored my simpler fix, that used the existing logic.
> >I don't like introducing this especially into the stable; much prefer
> >to go back to testing the flag as was being done before.
>
> Although your patch is technically sane, it depends on rtnl indirectly.
> My patch depends on rcu locking and synchronize_rcu which is direct.
> Therefore I think it is more appropriate.

After more review and thought I agree. But could we put some comments
in br_private.h to describe the dependency on ordering (synchronize_net).

Acked-by: Stephen Hemminger <stephen@networkplumber.org>