[PATCH net v2] tipc: fix use-after-free of the discoverer in tipc_disc_rcv()

[Weiming Shi <bestswngs@gmail.com>]

bearer_disable() frees b->disc with tipc_disc_delete()'s plain kfree(),
but tipc_disc_rcv() still dereferences b->disc in RX softirq under
rcu_read_lock() (tipc_udp_recv -> tipc_rcv -> tipc_disc_rcv).

L2 bearers are safe thanks to the synchronize_net() in
tipc_disable_l2_media(), but the UDP bearer defers that call to the
cleanup_bearer() workqueue, so the discoverer is freed with no grace
period:

 BUG: KASAN: slab-use-after-free in tipc_disc_rcv (net/tipc/discover.c:149)
 Read of size 8 at addr ffff88802348b728 by task poc_tipc/184
 <IRQ>
  tipc_disc_rcv (net/tipc/discover.c:149)
  tipc_rcv (net/tipc/node.c:2126)
  tipc_udp_recv (net/tipc/udp_media.c:391)
  udp_rcv (net/ipv4/udp.c:2643)
  ip_local_deliver_finish (net/ipv4/ip_input.c:241)
 </IRQ>
 Freed by task 181:
  kfree (mm/slub.c:6565)
  bearer_disable (net/tipc/bearer.c:418)
  tipc_nl_bearer_disable (net/tipc/bearer.c:1001)

The bearer is freed with kfree_rcu(); free the discoverer the same way.
Add an rcu_head to struct tipc_discoverer and free it and its skb from an
RCU callback.

Because the RCU callback (tipc_disc_free_rcu) lives in module text, a
call_rcu() that is still pending when the tipc module is unloaded would
invoke a freed function. Add an rcu_barrier() to tipc_exit() after the
bearer subsystem has been torn down, so all pending discoverer callbacks
have run before the module text goes away.

Reachable from an unprivileged user namespace: the TIPCv2 genl family is
netnsok and its bearer commands have no GENL_ADMIN_PERM. Needs CONFIG_TIPC
and CONFIG_TIPC_MEDIA_UDP.

Fixes: 25b0b9c4e835 ("tipc: handle collisions of 32-bit node address hash values")
Reported-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
v2:
 - split the over-80-column container_of() line (Tung Quang Nguyen)
 - add rcu_barrier() to tipc_exit() so a pending call_rcu() cannot fire
   into freed module text after rmmod (Eric Dumazet)

 net/tipc/core.c     |  3 +++
 net/tipc/discover.c | 14 ++++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/net/tipc/core.c b/net/tipc/core.c
index 434e70eabe08..747328e58d30 100644
--- a/net/tipc/core.c
+++ b/net/tipc/core.c
@@ -218,6 +218,9 @@ static void __exit tipc_exit(void)
 	unregister_pernet_device(&tipc_net_ops);
 	tipc_unregister_sysctl();
 
+	/* Wait for tipc_disc_free_rcu() callbacks queued from module text. */
+	rcu_barrier();
+
 	pr_info("Deactivated\n");
 }
 
diff --git a/net/tipc/discover.c b/net/tipc/discover.c
index 3e54d2df5683..696b7a8ed54d 100644
--- a/net/tipc/discover.c
+++ b/net/tipc/discover.c
@@ -58,6 +58,7 @@
  * @skb: request message to be (repeatedly) sent
  * @timer: timer governing period between requests
  * @timer_intv: current interval between requests (in ms)
+ * @rcu: RCU head for deferred freeing
  */
 struct tipc_discoverer {
 	u32 bearer_id;
@@ -69,6 +70,7 @@ struct tipc_discoverer {
 	struct sk_buff *skb;
 	struct timer_list timer;
 	unsigned long timer_intv;
+	struct rcu_head rcu;
 };
 
 /**
@@ -382,6 +384,15 @@ int tipc_disc_create(struct net *net, struct tipc_bearer *b,
 	return 0;
 }
 
+static void tipc_disc_free_rcu(struct rcu_head *rp)
+{
+	struct tipc_discoverer *d =
+		container_of(rp, struct tipc_discoverer, rcu);
+
+	kfree_skb(d->skb);
+	kfree(d);
+}
+
 /**
  * tipc_disc_delete - destroy object sending periodic link setup requests
  * @d: ptr to link dest structure
@@ -389,8 +400,7 @@ int tipc_disc_create(struct net *net, struct tipc_bearer *b,
 void tipc_disc_delete(struct tipc_discoverer *d)
 {
 	timer_shutdown_sync(&d->timer);
-	kfree_skb(d->skb);
-	kfree(d);
+	call_rcu(&d->rcu, tipc_disc_free_rcu);
 }
 
 /**
-- 
2.43.0