sigd_put_skb() delivers a signalling message to the daemon socket named
by the global @sigd pointer, ending in a call to sk_data_ready(). It
reads @sigd with no synchronisation, so it can race with a close of the
daemon socket: sigd_close() clears @sigd and the socket is then torn
down and freed.

Holding a reference on the socket is not enough to make this safe. The
daemon fd close runs __sock_release(), which frees the struct socket --
and the wait queue that sk->sk_wq points at -- via iput() once
->release() has returned. sk_data_ready() (sock_def_readable()) then
dereferences a freed sk_wq:

 KASAN: null-ptr-deref in range [0x0000000000000188-0x000000000000018f]
 RIP: 0010:sigd_put_skb (net/atm/signaling.c:65)
  sigd_enq2 (net/atm/signaling.c:228)
  sigd_enq (net/atm/signaling.c:237)
  svc_bind (net/atm/svc.c:135)
  __sys_bind
  __x64_sys_bind
  do_syscall_64

Fix it on both sides. sigd_close() now calls sock_orphan(), which under
sk_callback_lock sets SOCK_DEAD and clears sk_wq before the socket is
freed. sigd_put_skb() latches @sigd with READ_ONCE(), pins the socket
with find_get_vcc(), and then takes sk_callback_lock; it delivers only
while the socket is still the signalling daemon and not yet SOCK_DEAD,
otherwise it drops the skb. sk_callback_lock is used rather than
lock_sock() because sigd_put_skb() can be reached from vcc_sendmsg() ->
sigd_send(), which already holds lock_sock() on the daemon socket.

The READ_ONCE(sigd) != vcc check also rejects a reused VCC: a caller
preempted between latching @sigd and validating it in find_get_vcc()
could otherwise match a freed-and-reallocated, live (non-SOCK_DEAD) VCC.
This needs a concurrent (privileged) daemon teardown, so it is hardening.

Triggering the use-after-free requires CAP_NET_ADMIN and CAP_SYS_RAWIO
to attach the daemon.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Xiang Mei <redacted>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <redacted>
---
v2:
 - Also bail when READ_ONCE(sigd) != vcc, rejecting a reused VCC that a
   preempted caller could match in find_get_vcc() (hardening, no stable).

 net/atm/signaling.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/net/atm/signaling.c b/net/atm/signaling.c
index 358fbe5e4d1d0..346fb969166e8 100644
--- a/net/atm/signaling.c
+++ b/net/atm/signaling.c

@@ -54,14 +54,32 @@ static struct atm_vcc *find_get_vcc(struct atm_vcc *vcc)
 
 static void sigd_put_skb(struct sk_buff *skb)
 {
-	if (!sigd) {
+	struct atm_vcc *vcc;
+	struct sock *sk;
+
+	vcc = find_get_vcc(READ_ONCE(sigd));
+	if (!vcc) {
 		pr_debug("atmsvc: no signaling daemon\n");
 		kfree_skb(skb);
 		return;
 	}
-	atm_force_charge(sigd, skb->truesize);
-	skb_queue_tail(&sk_atm(sigd)->sk_receive_queue, skb);
-	sk_atm(sigd)->sk_data_ready(sk_atm(sigd));
+	sk = sk_atm(vcc);
+
+	/* Pairs with sock_orphan() in sigd_close(). */
+	read_lock_bh(&sk->sk_callback_lock);
+	/* Bail if torn down, or if the slot was reused by another VCC. */
+	if (sock_flag(sk, SOCK_DEAD) || READ_ONCE(sigd) != vcc) {
+		read_unlock_bh(&sk->sk_callback_lock);
+		sock_put(sk);
+		kfree_skb(skb);
+		return;
+	}
+	atm_force_charge(vcc, skb->truesize);
+	skb_queue_tail(&sk->sk_receive_queue, skb);
+	sk->sk_data_ready(sk);
+	read_unlock_bh(&sk->sk_callback_lock);
+
+	sock_put(sk);
 }
 
 static void modify_qos(struct atm_vcc *vcc, struct atmsvc_msg *msg)

@@ -257,6 +275,9 @@ static void sigd_close(struct atm_vcc *vcc)
 		pr_err("closing with requests pending\n");
 	skb_queue_purge(&sk_atm(vcc)->sk_receive_queue);
 
+	/* Make a concurrent sigd_put_skb() observe SOCK_DEAD and bail. */
+	sock_orphan(sk_atm(vcc));
+
 	read_lock(&vcc_sklist_lock);
 	for (i = 0; i < VCC_HTABLE_SIZE; ++i) {
 		struct hlist_head *head = &vcc_hash[i];

-- 
2.43.0