Re: [PATCH v1 net] bridge: Return an error when enabling STP in netns.

[Ido Schimmel <idosch@idosch.org>]

On Tue, Jul 11, 2023 at 04:54:15PM -0700, Kuniyuki Iwashima wrote:
> When we create an L2 loop on a bridge in netns, we will see packets storm
> even if STP is enabled.
> 
>   # unshare -n
>   # ip link add br0 type bridge
>   # ip link add veth0 type veth peer name veth1
>   # ip link set veth0 master br0 up
>   # ip link set veth1 master br0 up
>   # ip link set br0 type bridge stp_state 1
>   # ip link set br0 up
>   # sleep 30
>   # ip -s link show br0
>   2: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
>       link/ether b6:61:98:1c:1c:b5 brd ff:ff:ff:ff:ff:ff
>       RX: bytes  packets  errors  dropped missed  mcast
>       956553768  12861249 0       0       0       12861249  <-. Keep
>       TX: bytes  packets  errors  dropped carrier collsns     |  increasing
>       1027834    11951    0       0       0       0         <-'   rapidly
> 
> This is because llc_rcv() drops all packets in non-root netns and BPDU
> is dropped.
> 
> Let's show an error when enabling STP in netns.
> 
>   # unshare -n
>   # ip link add br0 type bridge
>   # ip link set br0 type bridge stp_state 1
>   Error: bridge: STP can't be enabled in non-root netns.
> 
> Note this commit will be reverted later when we namespacify the whole LLC
> infra.
> 
> Fixes: e730c15519d0 ("[NET]: Make packet reception network namespace safe")
> Suggested-by: Harry Coin <hcoin@quietfountain.com>

I'm not sure that's accurate. I read his response in the link below and
he says "I'd rather be warned than blocked" and "But better warned and
awaiting a fix than blocked", which I agree with. The patch has the
potential to cause a lot of regressions, but without actually fixing the
problem.

How about simply removing the error [1]? Since iproute2 commit
844c37b42373 ("libnetlink: Handle extack messages for non-error case"),
it can print extack warnings and not only errors. With the diff below:

 # unshare -n 
 # ip link add name br0 type bridge
 # ip link set dev br0 type bridge stp_state 1
 Warning: bridge: STP can't be enabled in non-root netns.
 # echo $?
 0

[1]
diff --git a/net/bridge/br_stp_if.c b/net/bridge/br_stp_if.c
index a807996ac56b..b5143de37938 100644
--- a/net/bridge/br_stp_if.c
+++ b/net/bridge/br_stp_if.c
@@ -201,10 +201,8 @@ int br_stp_set_enabled(struct net_bridge *br, unsigned long val,
 {
        ASSERT_RTNL();
 
-       if (!net_eq(dev_net(br->dev), &init_net)) {
+       if (!net_eq(dev_net(br->dev), &init_net))
                NL_SET_ERR_MSG_MOD(extack, "STP can't be enabled in non-root netns");
-               return -EINVAL;
-       }
 
        if (br_mrp_enabled(br)) {
                NL_SET_ERR_MSG_MOD(extack,

> Link: https://lore.kernel.org/netdev/0f531295-e289-022d-5add-5ceffa0df9bc@quietfountain.com/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> ---
>  net/bridge/br_stp_if.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/bridge/br_stp_if.c b/net/bridge/br_stp_if.c
> index 75204d36d7f9..a807996ac56b 100644
> --- a/net/bridge/br_stp_if.c
> +++ b/net/bridge/br_stp_if.c
> @@ -201,6 +201,11 @@ int br_stp_set_enabled(struct net_bridge *br, unsigned long val,
>  {
>  	ASSERT_RTNL();
>  
> +	if (!net_eq(dev_net(br->dev), &init_net)) {
> +		NL_SET_ERR_MSG_MOD(extack, "STP can't be enabled in non-root netns");
> +		return -EINVAL;
> +	}
> +
>  	if (br_mrp_enabled(br)) {
>  		NL_SET_ERR_MSG_MOD(extack,
>  				   "STP can't be enabled if MRP is already enabled");
> -- 
> 2.30.2
> 
>