[RFC PATCH] can: isotp: fix race between isotp_sendsmg() and isotp_release()

[Oliver Hartkopp <socketcan@hartkopp.net>]

As discussed with Dae R. Jeong and Hillf Danton here [1] the sendmsg()
function in isotp.c might get into a race condition when restoring the
former tx.state from the old_state. This patch removes the old_state
concept and implements a proper locking for ISOTP_IDLE transitions in
isotp_sendmsg() inspired by a simplification idea from Hillf Danton.
Additionally a new tx.state ISOTP_SHUTDOWN has been introduced to use
the same locking mechanism from isotp_release() which resolves a
potential race between isotp_sendsmg() and isotp_release().

[1] https://lore.kernel.org/linux-can/ZB%2F93xJxq%2FBUqAgG@dragonet/

Cc: Dae R. Jeong <threeearcat@gmail.com>
Cc: Hillf Danton <hdanton@sina.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
---
 net/can/isotp.c | 49 ++++++++++++++++++++++++++-----------------------
 1 file changed, 26 insertions(+), 23 deletions(-)

diff --git a/net/can/isotp.c b/net/can/isotp.c
index 9bc344851704..2ba043f5a9cb 100644
--- a/net/can/isotp.c
+++ b/net/can/isotp.c
@@ -117,11 +117,12 @@ MODULE_ALIAS("can-proto-6");
 enum {
 	ISOTP_IDLE = 0,
 	ISOTP_WAIT_FIRST_FC,
 	ISOTP_WAIT_FC,
 	ISOTP_WAIT_DATA,
-	ISOTP_SENDING
+	ISOTP_SENDING,
+	ISOTP_SHUTDOWN,
 };
 
 struct tpcon {
 	unsigned int idx;
 	unsigned int len;
@@ -878,12 +879,12 @@ static enum hrtimer_restart isotp_tx_timer_handler(struct hrtimer *hrtimer)
 {
 	struct isotp_sock *so = container_of(hrtimer, struct isotp_sock,
 					     txtimer);
 	struct sock *sk = &so->sk;
 
-	/* don't handle timeouts in IDLE state */
-	if (so->tx.state == ISOTP_IDLE)
+	/* don't handle timeouts in IDLE or SHUTDOWN state */
+	if (so->tx.state == ISOTP_IDLE || so->tx.state == ISOTP_SHUTDOWN)
 		return HRTIMER_NORESTART;
 
 	/* we did not get any flow control or echo frame in time */
 
 	/* report 'communication error on send' */
@@ -916,37 +917,37 @@ static enum hrtimer_restart isotp_txfr_timer_handler(struct hrtimer *hrtimer)
 
 static int isotp_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
 {
 	struct sock *sk = sock->sk;
 	struct isotp_sock *so = isotp_sk(sk);
-	u32 old_state = so->tx.state;
 	struct sk_buff *skb;
 	struct net_device *dev;
 	struct canfd_frame *cf;
 	int ae = (so->opt.flags & CAN_ISOTP_EXTEND_ADDR) ? 1 : 0;
 	int wait_tx_done = (so->opt.flags & CAN_ISOTP_WAIT_TX_DONE) ? 1 : 0;
 	s64 hrtimer_sec = ISOTP_ECHO_TIMEOUT;
 	int off;
 	int err;
 
-	if (!so->bound)
+	if (!so->bound || so->tx.state == ISOTP_SHUTDOWN)
 		return -EADDRNOTAVAIL;
 
+wait_free_buffer:
 	/* we do not support multiple buffers - for now */
-	if (cmpxchg(&so->tx.state, ISOTP_IDLE, ISOTP_SENDING) != ISOTP_IDLE ||
-	    wq_has_sleeper(&so->wait)) {
-		if (msg->msg_flags & MSG_DONTWAIT) {
-			err = -EAGAIN;
-			goto err_out;
-		}
+	if (wq_has_sleeper(&so->wait) && (msg->msg_flags & MSG_DONTWAIT))
+		return -EAGAIN;
 
-		/* wait for complete transmission of current pdu */
-		err = wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE);
-		if (err)
-			goto err_out;
+	/* wait for complete transmission of current pdu */
+	err = wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE);
+	if (err)
+		return err;
 
-		so->tx.state = ISOTP_SENDING;
+	if (cmpxchg(&so->tx.state, ISOTP_IDLE, ISOTP_SENDING) != ISOTP_IDLE) {
+		if (so->tx.state == ISOTP_SHUTDOWN)
+			return -EADDRNOTAVAIL;
+
+		goto wait_free_buffer;
 	}
 
 	if (!size || size > MAX_MSG_LENGTH) {
 		err = -EINVAL;
 		goto err_out_drop;
@@ -1072,25 +1073,24 @@ static int isotp_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
 		goto err_out_drop;
 	}
 
 	if (wait_tx_done) {
 		/* wait for complete transmission of current pdu */
-		wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE);
+		err = wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE);
+		if (err)
+			return err;
 
 		if (sk->sk_err)
 			return -sk->sk_err;
 	}
 
 	return size;
 
 err_out_drop:
 	/* drop this PDU and unlock a potential wait queue */
-	old_state = ISOTP_IDLE;
-err_out:
-	so->tx.state = old_state;
-	if (so->tx.state == ISOTP_IDLE)
-		wake_up_interruptible(&so->wait);
+	so->tx.state = ISOTP_IDLE;
+	wake_up_interruptible(&so->wait);
 
 	return err;
 }
 
 static int isotp_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
@@ -1147,15 +1147,18 @@ static int isotp_release(struct socket *sock)
 		return 0;
 
 	so = isotp_sk(sk);
 	net = sock_net(sk);
 
+wait_transmission_finish:
 	/* wait for complete transmission of current pdu */
 	wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE);
 
+	if (cmpxchg(&so->tx.state, ISOTP_IDLE, ISOTP_SHUTDOWN) != ISOTP_IDLE)
+		goto wait_transmission_finish;
+
 	/* force state machines to be idle also when a signal occurred */
-	so->tx.state = ISOTP_IDLE;
 	so->rx.state = ISOTP_IDLE;
 
 	spin_lock(&isotp_notifier_lock);
 	while (isotp_busy_notifier == so) {
 		spin_unlock(&isotp_notifier_lock);
-- 
2.30.2