[PATCH] [PATCH net] tipc: fix UAF race in tipc_mon_peer_up/down/remove_peer vs bearer teardown

[PATCH] [PATCH net] tipc: fix UAF race in tipc_mon_peer_up/down/remove_peer vs bearer teardown

[SnailSploit | Kai Aizen]

From: Kai Aizen <kai.aizen.dev@gmail.com>

CVE-2025-40280 fixed tipc_mon_reinit_self() accessing monitors[] from a
workqueue without RTNL.  That patch closed the workqueue path by adding
rtnl_lock() around the call.

However, three additional functions in the same subsystem access
tipc_net->monitors[] from softirq context with no RCU protection at all:

  tipc_mon_peer_up()      - called from tipc_node_write_unlock()
  tipc_mon_peer_down()    - called from tipc_node_write_unlock()
  tipc_mon_remove_peer()  - called from tipc_node_link_down()

These three are invoked from the packet receive path (tipc_rcv ->
tipc_node_write_unlock / tipc_node_link_down) and hold only the per-node
rwlock, not RTNL.

Concurrently, bearer_disable() -- which always holds RTNL per its own
inline documentation -- calls tipc_mon_delete(), which:

  1. acquires mon->lock
  2. sets tn->monitors[bearer_id] = NULL
  3. frees all peer entries
  4. releases mon->lock
  5. calls kfree(mon)                     <-- no synchronize_rcu()

The race is structural: there is no shared lock between the data-path
reader (which reads monitors[id] then acquires mon->lock) and the
teardown path (which acquires mon->lock, NULLs the slot, then frees).
A softirq thread can read a non-NULL mon pointer, get preempted, and
resume after kfree(mon) has run on another CPU, then call
write_lock_bh(&mon->lock) on freed memory:

  CPU 0 (softirq / tipc_rcv)            CPU 1 (RTNL / bearer_disable)
  tipc_mon_peer_up()
    mon = tipc_monitor(net, id)
    [mon is non-NULL]
                                         tipc_mon_delete()
                                           write_lock_bh(&mon->lock)
                                           tn->monitors[id] = NULL
                                           ...
                                           write_unlock_bh(&mon->lock)
                                           kfree(mon)
    write_lock_bh(&mon->lock)   <-- UAF

The fix mirrors the existing bearer_list[] pattern in the same module:
convert monitors[] to __rcu, use rcu_assign_pointer() on creation,
RCU_INIT_POINTER() + synchronize_rcu() on deletion (before the kfree),
and the appropriate rcu_dereference_bh() vs rtnl_dereference() variant
at each read site depending on execution context.

synchronize_rcu() in tipc_mon_delete() is placed after the
write_unlock_bh() and before timer_shutdown_sync() + kfree() to ensure
all softirq-context readers that already observed the old pointer have
completed before the memory is freed.

Fixes: 35c55c9877f8 ("tipc: add neighbor monitoring framework")
Cc: stable@vger.kernel.org
Signed-off-by: Kai Aizen <kai.aizen.dev@gmail.com>
---
 net/tipc/core.h    |  2 +-
 net/tipc/monitor.c | 45 +++++++++++++++++++++++++++++----------------
 2 files changed, 30 insertions(+), 17 deletions(-)

diff --git a/net/tipc/core.h b/net/tipc/core.h
index 9ce5f9ff6..cd582f7a2 100644
--- a/net/tipc/core.h
+++ b/net/tipc/core.h
@@ -109,7 +109,7 @@ struct tipc_net {
 	u32 num_links;
 
 	/* Neighbor monitoring list */
-	struct tipc_monitor *monitors[MAX_BEARERS];
+	struct tipc_monitor __rcu *monitors[MAX_BEARERS];
 	int mon_threshold;
 
 	/* Bearer list */
diff --git a/net/tipc/monitor.c b/net/tipc/monitor.c
index a94b9b36a..2a0665e1d 100644
--- a/net/tipc/monitor.c
+++ b/net/tipc/monitor.c
@@ -97,9 +97,21 @@ struct tipc_monitor {
 	unsigned long timer_intv;
 };
 
-static struct tipc_monitor *tipc_monitor(struct net *net, int bearer_id)
+/*
+ * tipc_monitor_rcu_bh - dereference monitors[] from softirq / data path.
+ * Caller must be in an RCU-bh read-side critical section (softirq context
+ * implicitly satisfies this on non-PREEMPT_RT kernels; use explicit
+ * rcu_read_lock_bh() where needed on RT).
+ */
+static struct tipc_monitor *tipc_monitor_rcu_bh(struct net *net, int bearer_id)
+{
+	return rcu_dereference_bh(tipc_net(net)->monitors[bearer_id]);
+}
+
+/* tipc_monitor_rtnl - dereference monitors[] from RTNL-held control path. */
+static struct tipc_monitor *tipc_monitor_rtnl(struct net *net, int bearer_id)
 {
-	return tipc_net(net)->monitors[bearer_id];
+	return rtnl_dereference(tipc_net(net)->monitors[bearer_id]);
 }
 
 const int tipc_max_domain_size = sizeof(struct tipc_mon_domain);
@@ -194,7 +206,7 @@ static struct tipc_peer *get_peer(struct tipc_monitor *mon, u32 addr)
 
 static struct tipc_peer *get_self(struct net *net, int bearer_id)
 {
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = tipc_monitor_rcu_bh(net, bearer_id);
 
 	return mon->self;
 }
@@ -351,7 +363,7 @@ static void mon_assign_roles(struct tipc_monitor *mon, struct tipc_peer *head)
 
 void tipc_mon_remove_peer(struct net *net, u32 addr, int bearer_id)
 {
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = tipc_monitor_rcu_bh(net, bearer_id);
 	struct tipc_peer *self;
 	struct tipc_peer *peer, *prev, *head;
 
@@ -421,7 +433,7 @@ static bool tipc_mon_add_peer(struct tipc_monitor *mon, u32 addr,
 
 void tipc_mon_peer_up(struct net *net, u32 addr, int bearer_id)
 {
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = tipc_monitor_rcu_bh(net, bearer_id);
 	struct tipc_peer *self = get_self(net, bearer_id);
 	struct tipc_peer *peer, *head;
 
@@ -440,7 +452,7 @@ void tipc_mon_peer_up(struct net *net, u32 addr, int bearer_id)
 
 void tipc_mon_peer_down(struct net *net, u32 addr, int bearer_id)
 {
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = tipc_monitor_rcu_bh(net, bearer_id);
 	struct tipc_peer *self;
 	struct tipc_peer *peer, *head;
 	struct tipc_mon_domain *dom;
@@ -480,7 +492,7 @@ void tipc_mon_peer_down(struct net *net, u32 addr, int bearer_id)
 void tipc_mon_rcv(struct net *net, void *data, u16 dlen, u32 addr,
 		  struct tipc_mon_state *state, int bearer_id)
 {
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = tipc_monitor_rcu_bh(net, bearer_id);
 	struct tipc_mon_domain *arrv_dom = data;
 	struct tipc_mon_domain dom_bef;
 	struct tipc_mon_domain *dom;
@@ -566,7 +578,7 @@ void tipc_mon_rcv(struct net *net, void *data, u16 dlen, u32 addr,
 void tipc_mon_prep(struct net *net, void *data, int *dlen,
 		   struct tipc_mon_state *state, int bearer_id)
 {
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = tipc_monitor_rcu_bh(net, bearer_id);
 	struct tipc_mon_domain *dom = data;
 	u16 gen = mon->dom_gen;
 	u16 len;
@@ -600,7 +612,7 @@ void tipc_mon_get_state(struct net *net, u32 addr,
 			struct tipc_mon_state *state,
 			int bearer_id)
 {
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = tipc_monitor_rcu_bh(net, bearer_id);
 	struct tipc_peer *peer;
 
 	if (!tipc_mon_is_active(net, mon)) {
@@ -651,7 +663,7 @@ int tipc_mon_create(struct net *net, int bearer_id)
 	struct tipc_peer *self;
 	struct tipc_mon_domain *dom;
 
-	if (tn->monitors[bearer_id])
+	if (rtnl_dereference(tn->monitors[bearer_id]))
 		return 0;
 
 	mon = kzalloc_obj(*mon, GFP_ATOMIC);
@@ -663,7 +675,7 @@ int tipc_mon_create(struct net *net, int bearer_id)
 		kfree(dom);
 		return -ENOMEM;
 	}
-	tn->monitors[bearer_id] = mon;
+	rcu_assign_pointer(tn->monitors[bearer_id], mon);
 	rwlock_init(&mon->lock);
 	mon->net = net;
 	mon->peer_cnt = 1;
@@ -682,7 +694,7 @@ int tipc_mon_create(struct net *net, int bearer_id)
 void tipc_mon_delete(struct net *net, int bearer_id)
 {
 	struct tipc_net *tn = tipc_net(net);
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = rtnl_dereference(tn->monitors[bearer_id]);
 	struct tipc_peer *self;
 	struct tipc_peer *peer, *tmp;
 
@@ -691,7 +703,7 @@ void tipc_mon_delete(struct net *net, int bearer_id)
 
 	self = get_self(net, bearer_id);
 	write_lock_bh(&mon->lock);
-	tn->monitors[bearer_id] = NULL;
+	RCU_INIT_POINTER(tn->monitors[bearer_id], NULL);
 	list_for_each_entry_safe(peer, tmp, &self->list, list) {
 		list_del(&peer->list);
 		hlist_del(&peer->hash);
@@ -700,6 +712,7 @@ void tipc_mon_delete(struct net *net, int bearer_id)
 	}
 	mon->self = NULL;
 	write_unlock_bh(&mon->lock);
+	synchronize_rcu();
 	timer_shutdown_sync(&mon->timer);
 	kfree(self->domain);
 	kfree(self);
@@ -712,7 +725,7 @@ void tipc_mon_reinit_self(struct net *net)
 	int bearer_id;
 
 	for (bearer_id = 0; bearer_id < MAX_BEARERS; bearer_id++) {
-		mon = tipc_monitor(net, bearer_id);
+		mon = rtnl_dereference(tipc_net(net)->monitors[bearer_id]);
 		if (!mon)
 			continue;
 		write_lock_bh(&mon->lock);
@@ -798,7 +811,7 @@ static int __tipc_nl_add_monitor_peer(struct tipc_peer *peer,
 int tipc_nl_add_monitor_peer(struct net *net, struct tipc_nl_msg *msg,
 			     u32 bearer_id, u32 *prev_node)
 {
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = rtnl_dereference(tipc_net(net)->monitors[bearer_id]);
 	struct tipc_peer *peer;
 
 	if (!mon)
@@ -827,7 +840,7 @@ int tipc_nl_add_monitor_peer(struct net *net, struct tipc_nl_msg *msg,
 int __tipc_nl_add_monitor(struct net *net, struct tipc_nl_msg *msg,
 			  u32 bearer_id)
 {
-	struct tipc_monitor *mon = tipc_monitor(net, bearer_id);
+	struct tipc_monitor *mon = rtnl_dereference(tipc_net(net)->monitors[bearer_id]);
 	char bearer_name[TIPC_MAX_BEARER_NAME];
 	struct nlattr *attrs;
 	void *hdr;
-- 
2.43.0

[syzbot ci] Re: [PATCH net] tipc: fix UAF race in tipc_mon_peer_up/down/remove_peer vs bearer teardown

[syzbot ci]

syzbot ci has tested the following series

[v1] [PATCH net] tipc: fix UAF race in tipc_mon_peer_up/down/remove_peer vs bearer teardown
https://lore.kernel.org/all/20260415061211.45530-1-95986478+SnailSploit@users.noreply.github.com
* [PATCH] [PATCH net] tipc: fix UAF race in tipc_mon_peer_up/down/remove_peer vs bearer teardown

and found the following issue:
WARNING: suspicious RCU usage in tipc_mon_delete

Full report is available here:
https://ci.syzbot.org/series/6267bc07-4172-4821-b3e5-dac381479d9d

***

WARNING: suspicious RCU usage in tipc_mon_delete

tree:      net-next
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net-next.git
base:      35c2c39832e569449b9192fa1afbbc4c66227af7
arch:      amd64
compiler:  Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
config:    https://ci.syzbot.org/builds/a29dabe7-96d8-4072-bc2c-d798a349301e/config
syz repro: https://ci.syzbot.org/findings/f144d75a-7c29-41a1-988e-09892a89baa1/syz_repro

tipc: Disabling bearer <eth:syzkaller0>
=============================
WARNING: suspicious RCU usage
syzkaller #0 Not tainted
-----------------------------
net/tipc/monitor.c:108 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz.2.19/5962:
 #0: ffffffff8fbcba48 (rtnl_mutex){+.+.}-{4:4}, at: tun_detach drivers/net/tun.c:634 [inline]
 #0: ffffffff8fbcba48 (rtnl_mutex){+.+.}-{4:4}, at: tun_chr_close+0x3e/0x1c0 drivers/net/tun.c:3438

stack backtrace:
CPU: 1 UID: 0 PID: 5962 Comm: syz.2.19 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 lockdep_rcu_suspicious+0x13f/0x1d0 kernel/locking/lockdep.c:6876
 tipc_monitor_rcu_bh+0xf5/0x110 net/tipc/monitor.c:108
 get_self net/tipc/monitor.c:209 [inline]
 tipc_mon_delete+0x10b/0x4d0 net/tipc/monitor.c:704
 tipc_l2_device_event+0x370/0x680 net/tipc/bearer.c:-1
 notifier_call_chain+0x1be/0x400 kernel/notifier.c:85
 call_netdevice_notifiers_extack net/core/dev.c:2287 [inline]
 call_netdevice_notifiers net/core/dev.c:2301 [inline]
 unregister_netdevice_many_notify+0x17a5/0x22c0 net/core/dev.c:12464
 unregister_netdevice_many net/core/dev.c:12527 [inline]
 unregister_netdevice_queue+0x31f/0x360 net/core/dev.c:12337
 unregister_netdevice include/linux/netdevice.h:3427 [inline]
 __tun_detach+0x6d9/0x15d0 drivers/net/tun.c:621
 tun_detach drivers/net/tun.c:637 [inline]
 tun_chr_close+0x10a/0x1c0 drivers/net/tun.c:3438
 __fput+0x44f/0xa70 fs/file_table.c:469
 task_work_run+0x1d9/0x270 kernel/task_work.c:233
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 __exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
 exit_to_user_mode_loop+0xed/0x480 kernel/entry/common.c:98
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
 syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:325 [inline]
 do_syscall_64+0x32d/0xf80 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7b26d9c819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffec30cee78 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 00007ffec30cef60 RCX: 00007f7b26d9c819
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 0000000000011900 R08: 0000000000000001 R09: 0000000000000000
R10: 0000001b2e520000 R11: 0000000000000246 R12: 00007ffec30cefa0
R13: 00007f7b27015fac R14: 000000000001193b R15: 00007f7b27015fa0
 </TASK>


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: syzbot@syzkaller.appspotmail.com

---
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