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 <redacted>
Link: https://lore.kernel.org/netdev/0f531295-e289-022d-5add-5ceffa0df9bc@quietfountain.com/ (local)
Signed-off-by: Kuniyuki Iwashima <redacted>
---
 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");