[STABLE v6.1-v5.10 2/2] Bluetooth: L2CAP: refuse __l2cap_chan_add if chan already has conn

From: Pauli Virtanen <redacted>

[ Upstream commit 86b55991d3cd690e73c5ef8d20b75ae4905b357a ]

l2cap_chan may be linked to l2cap_conn at most once. This is assumed in
several places, eg l2cap_chan_del cleanup.

There is a TOCTOU race where the invariant is violated:

    [Task 1]                          [Task 2]
    l2cap_chan_connect                l2cap_sock_bind
      l2cap_chan_lock                   lock_sock
      l2cap_state_change                if (sk->sk_state != BT_OPEN)
        chan->state = BT_CONNECT
        l2cap_sock_state_change_cb      chan->state = BT_BOUND
                                        sk->sk_state = BT_BOUND
          lock_sock <------------------ release_sock
          sk->sk_state = BT_CONNECT

l2cap_sock_connect() does not check sk->sk_state, so since chan->state
is now BT_BOUND, subsequent connect() ends up with second
__l2cap_chan_add.

Explicitly document and check the invariant in __l2cap_chan_add with
WARN_ON_ONCE. The only callsite where it could be hit is
l2cap_chan_connect, so add pre-check there to avoid relying on
chan->state. chan->state read/write is not properly guarded currently so
there can be other TOCTOUC problems.

Add l2cap_lock_chan in l2cap_sock_bind() to guard chan->state write.

Fixes: b66774b48dd9 ("Bluetooth: L2CAP: Fix UAF in channel timeout by holding conn ref")
Assisted-by: deepseek-4-flash # finding the race condition
Signed-off-by: Pauli Virtanen <redacted>
Signed-off-by: Luiz Augusto von Dentz <redacted>
[Lee: Resolve context conflict around mutex_lock(&conn->chan_lock) prior to commit 56c2b5831d39 ("Bluetooth: L2CAP: fix out-of-bounds write in l2cap_ecred_connect")]
Signed-off-by: Lee Jones <lee@kernel.org>
---
 net/bluetooth/l2cap_core.c | 7 ++++++-
 net/bluetooth/l2cap_sock.c | 2 ++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 88ddd8eabae9..445a43c721f6 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c

@@ -630,6 +630,10 @@ void __l2cap_chan_add(struct l2cap_conn *conn, struct l2cap_chan *chan)
 	BT_DBG("conn %p, psm 0x%2.2x, dcid 0x%4.4x", conn,
 	       __le16_to_cpu(chan->psm), chan->dcid);
 
+	/* Caller must ensure l2cap_chan is linked to l2cap_conn only once */
+	if (WARN_ON_ONCE(chan->conn || test_bit(FLAG_DEL, &chan->flags)))
+		return;
+
 	conn->disc_reason = HCI_ERROR_REMOTE_USER_TERM;
 
 	chan->conn = l2cap_conn_get(conn);

@@ -7431,7 +7435,8 @@ int l2cap_chan_connect(struct l2cap_chan *chan, __le16 psm, u16 cid,
 	mutex_lock(&conn->chan_lock);
 	l2cap_chan_lock(chan);
 
-	if (cid && __l2cap_get_chan_by_dcid(conn, cid)) {
+	if ((cid && __l2cap_get_chan_by_dcid(conn, cid)) || chan->conn ||
+	    test_bit(FLAG_DEL, &chan->flags)) {
 		hci_conn_drop(hcon);
 		err = -EBUSY;
 		goto chan_unlock;

diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index b11558b0dfec..655922aa82f1 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c

@@ -111,6 +111,7 @@ static int l2cap_sock_bind(struct socket *sock, struct sockaddr *addr, int alen)
 			return -EINVAL;
 	}
 
+	l2cap_chan_lock(chan);
 	lock_sock(sk);
 
 	if (sk->sk_state != BT_OPEN) {

@@ -176,6 +177,7 @@ static int l2cap_sock_bind(struct socket *sock, struct sockaddr *addr, int alen)
 
 done:
 	release_sock(sk);
+	l2cap_chan_unlock(chan);
 	return err;
 }
 

-- 
2.56.0.307.g5fe7bd680c-goog