[PATCH net v2 2/2] net: bridge: fix MST static key usage

[Nikolay Aleksandrov]

As Ido pointed out, the static key usage in MST is buggy and should use
inc/dec instead of enable/disable because we can have multiple bridges
with MST enabled which means a single bridge can disable MST for all.
Use static_branch_inc/dec to avoid that. When destroying a bridge decrement
the key if MST was enabled.

Fixes: ec7328b59176 ("net: bridge: mst: Multiple Spanning Tree (MST) mode")
Reported-by: Ido Schimmel <ido...@nvidia.com>
Closes: 
https://lore.kernel.org/netdev/20251104120313.1306566-1-ra...@blackwall.org/T/#m6888d87658f94ed1725433940f4f4ebb00b5a68b
Signed-off-by: Nikolay Aleksandrov <ra...@blackwall.org>
---
v2: new fix

 net/bridge/br_if.c      |  1 +
 net/bridge/br_mst.c     | 10 ++++++++--
 net/bridge/br_private.h |  5 +++++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/net/bridge/br_if.c b/net/bridge/br_if.c
index 98c5b9c3145f..ca3a637d7cca 100644
--- a/net/bridge/br_if.c
+++ b/net/bridge/br_if.c
@@ -386,6 +386,7 @@ void br_dev_delete(struct net_device *dev, struct list_head 
*head)
                del_nbp(p);
        }
 
+       br_mst_uninit(br);
        br_recalculate_neigh_suppress_enabled(br);
 
        br_fdb_delete_by_port(br, NULL, 0, 1);
diff --git a/net/bridge/br_mst.c b/net/bridge/br_mst.c
index 3f24b4ee49c2..43a300ae6bfa 100644
--- a/net/bridge/br_mst.c
+++ b/net/bridge/br_mst.c
@@ -22,6 +22,12 @@ bool br_mst_enabled(const struct net_device *dev)
 }
 EXPORT_SYMBOL_GPL(br_mst_enabled);
 
+void br_mst_uninit(struct net_bridge *br)
+{
+       if (br_opt_get(br, BROPT_MST_ENABLED))
+               static_branch_dec(&br_mst_used);
+}
+
 int br_mst_get_info(const struct net_device *dev, u16 msti, unsigned long 
*vids)
 {
        const struct net_bridge_vlan_group *vg;
@@ -225,9 +231,9 @@ int br_mst_set_enabled(struct net_bridge *br, bool on,
                return err;
 
        if (on)
-               static_branch_enable(&br_mst_used);
+               static_branch_inc(&br_mst_used);
        else
-               static_branch_disable(&br_mst_used);
+               static_branch_dec(&br_mst_used);
 
        br_opt_toggle(br, BROPT_MST_ENABLED, on);
        return 0;
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index b571d6f61389..7280c4e9305f 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -1954,6 +1954,7 @@ int br_mst_fill_info(struct sk_buff *skb,
                     const struct net_bridge_vlan_group *vg);
 int br_mst_process(struct net_bridge_port *p, const struct nlattr *mst_attr,
                   struct netlink_ext_ack *extack);
+void br_mst_uninit(struct net_bridge *br);
 #else
 static inline bool br_mst_is_enabled(const struct net_bridge_port *p)
 {
@@ -1989,6 +1990,10 @@ static inline int br_mst_process(struct net_bridge_port 
*p,
 {
        return -EOPNOTSUPP;
 }
+
+static inline void br_mst_uninit(struct net_bridge *br)
+{
+}
 #endif
 
 struct nf_br_ops {
-- 
2.51.0