Re: [PATCH] net bridge: add null pointer check, fix panic

Alexander Y. Fomichev
Mon, 11 Nov 2013 10:27:21 -0000

On Thu, Jun 20, 2013 at 11:29 AM, Eric Dumazet <eric.dumazet@gmail.com>
wrote:
> On Thu, 2013-06-20 at 15:00 +0800, xiaoming gao wrote:
>
>> HI Eric
>> the problem is as follow:
>> br_del_if()-->del_nbp():
>>
>> list_del_rcu(&p->list);
>> dev->priv_flags &= ~IFF_BRIDGE_PORT;
>>
>> ------>at this point, the nic be deleting still have rx_handler , so
, may in br_handle_frame()
>> ------>br_port_exists() will return false,so br_get_port_rcu() will
return NULL
>> ------>so in br_handle_frame , there will be a null panic.
>>
>> netdev_rx_handler_unregister(dev);
>> synchronize_net();
>
> This code is no longer like that in current tree.
> Check commit 4a0b5ec12f0ffc3024616e6dc62cf8a04c54edcd
> ("bridge: remove a redundant synchronize_net()")
>
>>
>>
>> i have checked commit 00cfec37484761a44, i think it didn''t fix
this bug..
>
> I claim adding NULL tests is not needed in the fast path, it was clearly
> stated in the changelog.
>
> I would change the dismantle path to make sure br_get_port_rcu() does
> not return NULL in the fast path, and remove the test on FF_BRIDGE_PORT
> Try something like that :
>
> diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
> index 1b8b8b8..2edfb80 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 = __br_port_get_rcu(skb->dev);
>         struct net_bridge *br;
>         struct net_bridge_fdb_entry *dst;
>         struct net_bridge_mdb_entry *mdst;
> @@ -68,7 +68,7 @@ int br_handle_frame_finish(struct sk_buff *skb)
>         bool unicast = true;
>         u16 vid = 0;
>
> -       if (!p || p->state == BR_STATE_DISABLED)
> +       if (p->state == BR_STATE_DISABLED)
>                 goto drop;
>
>         if (!br_allowed_ingress(p->br, nbp_get_vlan_info(p), skb,
&vid))
> @@ -142,7 +142,7 @@ 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 = __br_port_get_rcu(skb->dev);
>         u16 vid = 0;
>
>         br_vlan_get_tag(skb, &vid);
> @@ -172,7 +172,7 @@ 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 = __br_port_get_rcu(skb->dev);
>
>         if (unlikely(is_link_local_ether_addr(dest))) {
>                 /*
> diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
> index 3be89b3..9fdd467 100644
> --- a/net/bridge/br_private.h
> +++ b/net/bridge/br_private.h
> @@ -184,6 +184,11 @@ struct net_bridge_port
>
>  #define br_port_exists(dev) (dev->priv_flags & IFF_BRIDGE_PORT)
>
> +static inline struct net_bridge_port *__br_port_get_rcu(const struct
net_device *dev)
> +{
> +       return rcu_dereference(dev->rx_handler_data);
> +}
> +
>  static inline struct net_bridge_port *br_port_get_rcu(const struct
net_device *dev)
>  {
>         struct net_bridge_port *port >
>
> --
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> I claim adding NULL tests is not needed in the fast path, it was clearly
> stated in the changelog.

Hello,

This commit makes trouble for current STP.
Two days ago i tried to switch to 3.10.18 and i''ve caught "bad
magic"
on uninitialized br->lock:
./net/bridge/br_stp_bpdu.c +158 in br_stp_rcv (trace attached):

        p = br_port_get_rcu(dev);

        br = p->br;
        spin_lock(&br->lock); <- here
-----------------------------------------------

 Bisect pointed to this commit
(linux-stable: 960b8e5018a552f62cfbc0dfe94be7b6ba178f13)
(mainline 716ec052d2280d511e10e90ad54a86f5b5d4dcc2)

As far as i can see this happens when:

a) bridge module had been loaded but there was no bridge interface,
br->lock had not been initialized.
b) interface had been in promiscuous mod (tcpdump)
c) stp broadcasts 01:80:c2:00:00:00 coming to this iface
   (llc_rcv drops PACKET_OTHERHOST to protect us in promiscuous mode
but seems like not a broadcasts)
d) and finally rx_handler_data had been initialised for this interface
( by macvlan in my case)

It seems like STP needs its own IFF_BRIDGE_PORT check.
probably an easiest option to check it in br_stp_rcv as before (or
probably in llc_rcv)...

--
Best regards.
       Alexander Y. Fomichev <git.user@gmail.com>