RE: [PATCH net] tipc: read le->link under the node lock in tipc_node_link_down()
From: Tung Quang Nguyen <hidden>
Date: 2026-08-03 02:03:33
Also in:
lkml
Subject: [PATCH net] tipc: read le->link under the node lock in
tipc_node_link_down()
From: Jun Yang <redacted>
tipc_node_link_down() caches the link pointer before taking n->lock:
struct tipc_link *l = le->link; /* unlocked */
if (!l)
return;
tipc_node_write_lock(n);
if (!tipc_link_is_establishing(l)) { /* deref l */
...
tipc_link_reset(l); /* write into l */
if (delete) {
kfree(l);
le->link = NULL;
The delete=true caller frees that very object under n->lock, so the lock does
not protect the cached pointer against it:
- CPU A, delete=false: tipc_rcv() on TIPC_LINK_DOWN_EVT, or the link
supervision timer via tipc_node_timeout(), reads l unlocked and then
dereferences it under n->lock;
- CPU B, delete=true: netlink TIPC_NL_BEARER_DISABLE -> bearer_disable()
-> tipc_node_delete_links() -> tipc_node_link_down(n, bearer_id, true)
-> kfree(l).
The link is freed with plain kfree(), not kfree_rcu(), and for UDP bearers
disable_media() only schedules the asynchronous cleanup_bearer() work, so
its synchronize_net() runs after the links are already gone. An in-flight CPU A
that has read l therefore dereferences freed memory once B frees it:
a use-after-free read in tipc_link_is_establishing(), and a use-after-free write
via tipc_link_reset() on the establishing branch.
BUG: KASAN: slab-use-after-free in tipc_link_is_establishing+0x3f/0x50
Read of size 4 at addr ffff888111f90868 by task swapper/3/0
tipc_link_is_establishing+0x3f/0x50
tipc_node_link_down+0x1a0/0x450
tipc_node_timeout+0x63d/0xca0
Allocated by task 7903:
tipc_link_create+0x17e/0xf20
tipc_node_check_dest+0x98b/0x1150
tipc_disc_rcv+0x9f3/0x10c0
tipc_udp_recv+0x42a/0x7e0
Freed by task 7903:
tipc_node_link_down+0x301/0x450
tipc_node_delete_links+0xf5/0x240
bearer_disable+0xc0/0x240
__tipc_nl_bearer_disable+0x21f/0x380Please update the commit message with decoded stack trace (using linux/scripts/decode_stacktrace.sh).
quoted hunk ↗ jump to hunk
Move the le->link read inside tipc_node_write_lock(), so it is serialised against the kfree() in the delete path. A racing teardown now either has not run yet, and we see a valid link, or has already run, and we see NULL. Fixes: 73f646cec354 ("tipc: delay ESTABLISH state event when link is established") Cc: stable@kernel.org Reported-by: TencentOS Corvus AI <redacted> Signed-off-by: Jun Yang <redacted> --- net/tipc/node.c | 15 +++++++++++---- 1 file changed, 11 insertions(+), 4 deletions(-)diff --git a/net/tipc/node.c b/net/tipc/node.c index8e4ef2630ae4..918e59362d07 100644--- a/net/tipc/node.c +++ b/net/tipc/node.c@@ -1063,16 +1063,23 @@ static void tipc_node_link_down(struct tipc_node*n, int bearer_id, bool delete) { struct tipc_link_entry *le = &n->links[bearer_id];
Above node access also needs to be moved to below ' tipc_node_write_lock()'.
struct tipc_media_addr *maddr = NULL;
- struct tipc_link *l = le->link;
int old_bearer_id = bearer_id;
struct sk_buff_head xmitq;
-
- if (!l)
- return;
+ struct tipc_link *l;
__skb_queue_head_init(&xmitq);
+ /* Read le->link under n->lock: the delete path below kfree()s it while
+ * holding the same lock, so an unlocked read here could be raced by a
+ * concurrent bearer teardown and leave us with a freed pointer.
+ */
tipc_node_write_lock(n);
+ l = le->link;
+ if (!l) {
+ tipc_node_write_unlock(n);Please use 'tipc_node_write_unlock_fast()' instead.
+ return;
+ }
+
if (!tipc_link_is_establishing(l)) {
__tipc_node_link_down(n, &bearer_id, &xmitq, &maddr);
} else {
--
2.55.0