linux-kernel - [PATCH] bridge: Fix a deadlock when enabling multicast snooping

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Date:   Tue, 1 Dec 2020 16:40:47 -0500
From:   Joseph Huang <Joseph.Huang@...min.com>
To:     Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <nikolay@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        <bridge@...ts.linux-foundation.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
CC:     Joseph Huang <Joseph.Huang@...min.com>
Subject: [PATCH] bridge: Fix a deadlock when enabling multicast snooping

When enabling multicast snooping, bridge module deadlocks on multicast_lock
if 1) IPv6 is enabled, and 2) there is an existing querier on the same L2
network.

The deadlock was caused by the following sequence: While holding the lock,
br_multicast_open calls br_multicast_join_snoopers, which eventually causes
IP stack to (attempt to) send out a Listener Report (in igmp6_join_group).
Since the destination Ethernet address is a multicast address, br_dev_xmit
feeds the packet back to the bridge via br_multicast_rcv, which in turn
calls br_multicast_add_group, which then deadlocks on multicast_lock.

The fix is to move the call br_multicast_join_snoopers outside of the
critical section. This works since br_multicast_join_snoopers only deals
with IP and does not modify any multicast data structures of the bridge,
so there's no need to hold the lock.

Fixes: 4effd28c1245 ("bridge: join all-snoopers multicast address")

Signed-off-by: Joseph Huang <Joseph.Huang@...min.com>
---
 net/bridge/br_device.c    | 3 +++
 net/bridge/br_multicast.c | 8 ++++----
 net/bridge/br_private.h   | 5 +++++
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
index 7730c8f3cb53..8d3ec29d3875 100644
--- a/net/bridge/br_device.c
+++ b/net/bridge/br_device.c
@@ -177,6 +177,9 @@ static int br_dev_open(struct net_device *dev)
 	br_stp_enable_bridge(br);
 	br_multicast_open(br);
 
+	if (br_opt_get(br, BROPT_MULTICAST_ENABLED))
+		br_multicast_join_snoopers(br);
+
 	return 0;
 }
 
diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index eae898c3cff7..6c8d6b2eed91 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -3286,7 +3286,7 @@ static inline void br_ip6_multicast_join_snoopers(struct net_bridge *br)
 }
 #endif
 
-static void br_multicast_join_snoopers(struct net_bridge *br)
+void br_multicast_join_snoopers(struct net_bridge *br)
 {
 	br_ip4_multicast_join_snoopers(br);
 	br_ip6_multicast_join_snoopers(br);
@@ -3336,9 +3336,6 @@ static void __br_multicast_open(struct net_bridge *br,
 
 void br_multicast_open(struct net_bridge *br)
 {
-	if (br_opt_get(br, BROPT_MULTICAST_ENABLED))
-		br_multicast_join_snoopers(br);
-
 	__br_multicast_open(br, &br->ip4_own_query);
 #if IS_ENABLED(CONFIG_IPV6)
 	__br_multicast_open(br, &br->ip6_own_query);
@@ -3509,6 +3506,9 @@ int br_multicast_toggle(struct net_bridge *br, unsigned long val)
 unlock:
 	spin_unlock_bh(&br->multicast_lock);
 
+	if (br_opt_get(br, BROPT_MULTICAST_ENABLED))
+		br_multicast_join_snoopers(br);
+
 	return 0;
 }
 
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index 345118e35c42..7085dd747130 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -792,6 +792,7 @@ void br_multicast_del_port(struct net_bridge_port *port);
 void br_multicast_enable_port(struct net_bridge_port *port);
 void br_multicast_disable_port(struct net_bridge_port *port);
 void br_multicast_init(struct net_bridge *br);
+void br_multicast_join_snoopers(struct net_bridge *br);
 void br_multicast_open(struct net_bridge *br);
 void br_multicast_stop(struct net_bridge *br);
 void br_multicast_dev_del(struct net_bridge *br);
@@ -969,6 +970,10 @@ static inline void br_multicast_init(struct net_bridge *br)
 {
 }
 
+static inline void br_multicast_join_snoopers(struct net_bridge *br)
+{
+}
+
 static inline void br_multicast_open(struct net_bridge *br)
 {
 }
-- 
2.29.2