Subject: Re: [PATCH net-next] bridge: fix NULL pointer deref in br_handle_frame

From: Stephen Hemminger <[email-masked]>
Date: Fri, 13 Sep 2013 09:21:51 -0700

On Fri, 13 Sep 2013 11:09:47 +0800
Hong Zhiguo <[email-masked]> wrote:

> From: Hong Zhiguo <[email-masked]>
> 
> I got an Oops on my box when br_handle_frame is called between these
> 2 lines of del_nbp:
> 	dev->priv_flags &= ~IFF_BRIDGE_PORT;
> 	/* --> br_handle_frame is called at this time */
> 	netdev_rx_handler_unregister(dev);
> 
> In br_handle_frame the return of br_port_get_rcu(dev) is dereferenced
> without check but br_port_get_rcu(dev) returns NULL if:
> 	!(dev->priv_flags & IFF_BRIDGE_PORT)
> 
> In my first fix I moved netdev_rx_handler_unregister up. Eric Dumazet
> pointed out the testing of IFF_BRIDGE_PORT is not necessary here since
> we're in rcu_read_lock and we have synchronize_net() in
> netdev_rx_handler_unregister. This fix removed the testing of
> IFF_BRIDGE_PORT.
> 
> I tested the fix on my box with script doing "brctl addif" and "brctl
> delif" repeatedly while a lot of broadcast frame present on the LAN.
> I added msleep in del_nbp between setting of priv_flags and unregister
> so it's easy to reproduce the oops without the fix.
> 
> I'll send another patch to net-next to take care of br_netfilter and
> ebtable if necessary(seems there's NULL check following but I'll
> have a look).
> 
> The Oops(some lines omitted):
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000021
> IP: [<ffffffff8150901d>] br_handle_frame+0xed/0x230
> Oops: 0000 [#1] PREEMPT SMP
> RIP: 0010:[<ffffffff8150901d>]  [<ffffffff8150901d>] br_handle_frame+0xed/0x230
> RSP: 0018:ffff880030403c10  EFLAGS: 00010286
> Stack:
>  ffff88002c945700 ffffffff81508f30 0000000000000000 ffff88002d41e000
>  ffff880030403c98 ffffffff81477acb ffffffff81477821 ffff880030403c68
>  ffffffff81090e10 00ff88002d545c80 ffff88002c945700 ffffffff81aa50c0
> Call Trace:
>  <IRQ>
>  [<ffffffff81508f30>] ? br_handle_frame_finish+0x300/0x300
>  [<ffffffff81477acb>] __netif_receive_skb_core+0x39b/0x880
> 
> Signed-off-by: Hong Zhiguo <[email-masked]>
> ---
>  net/bridge/br_input.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
> index a2fd37e..2244049 100644
> --- a/net/bridge/br_input.c
> +++ b/net/bridge/br_input.c
> @@ -60,7 +60,7 @@ static int br_pass_frame_up(struct sk_buff *skb)
>  int br_handle_frame_finish(struct sk_buff *skb)
>  {
>  	const unsigned char *dest = eth_hdr(skb)->h_dest;
> -	struct net_bridge_port *p = br_port_get_rcu(skb->dev);
> +	struct net_bridge_port *p = rcu_dereference(skb->dev->rx_handler_data);
>  	struct net_bridge *br;
>  	struct net_bridge_fdb_entry *dst;
>  	struct net_bridge_mdb_entry *mdst;
> @@ -143,7 +143,7 @@ drop:
>  /* note: already called with rcu_read_lock */
>  static int br_handle_local_finish(struct sk_buff *skb)
>  {
> -	struct net_bridge_port *p = br_port_get_rcu(skb->dev);
> +	struct net_bridge_port *p = rcu_dereference(skb->dev->rx_handler_data);
>  	u16 vid = 0;
>  
>  	br_vlan_get_tag(skb, &vid);
> @@ -173,7 +173,7 @@ rx_handler_result_t br_handle_frame(struct sk_buff **pskb)
>  	if (!skb)
>  		return RX_HANDLER_CONSUMED;
>  
> -	p = br_port_get_rcu(skb->dev);
> +	p = rcu_dereference(skb->dev->rx_handler_data);
>  
>  	if (unlikely(is_link_local_ether_addr(dest))) {
>  		/*

There are more uses of br_port_get_rcu that have the same problem.
For example receiving an STP packet in that window.
I bet if you look at all the callers of br_port_get_rcu() you will
see the same issue, therefore either the check should be removed from br_port_get_rcu.

A little bit of history, the bridge code orginally did not use RCU,
and there was a flag on the device. Then RCU was added, then the receive
handler stuff was added.