[PATCH] can: isotp: fix error path in isotp_sendmsg() to unlock wait queue

[Oliver Hartkopp <socketcan@xxxxxxxxxxxx>]

Commit 43a08c3bdac4 ("can: isotp: isotp_sendmsg(): fix TX buffer concurrent
access in isotp_sendmsg()") introduced a new locking scheme that may render
the userspace application in a locking state when an error is detected.
This issue shows up under high load on simultaneously running isotp channels
with identical configuration which is against the ISO specification and
therefore breaks any reasonable PDU communication anyway.

Fixes: 43a08c3bdac4 ("can: isotp: isotp_sendmsg(): fix TX buffer concurrent access in isotp_sendmsg()")
Link: https://lore.kernel.org/all/20220209073601.25728-1-socketcan@xxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Cc: Ziyang Xuan <william.xuanziyang@xxxxxxxxxx>
Signed-off-by: Oliver Hartkopp <socketcan@xxxxxxxxxxxx>
Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>

diff --git a/net/can/isotp.c b/net/can/isotp.c
index 9149e8d8aefc..d2a430b6a13b 100644
--- a/net/can/isotp.c
+++ b/net/can/isotp.c
@@ -887,7 +887,7 @@ static int isotp_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
 
 	if (!size || size > MAX_MSG_LENGTH) {
 		err = -EINVAL;
-		goto err_out;
+		goto err_out_drop;
 	}
 
 	/* take care of a potential SF_DL ESC offset for TX_DL > 8 */
@@ -897,24 +897,24 @@ static int isotp_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
 	if ((so->opt.flags & CAN_ISOTP_SF_BROADCAST) &&
 	    (size > so->tx.ll_dl - SF_PCI_SZ4 - ae - off)) {
 		err = -EINVAL;
-		goto err_out;
+		goto err_out_drop;
 	}
 
 	err = memcpy_from_msg(so->tx.buf, msg, size);
 	if (err < 0)
-		goto err_out;
+		goto err_out_drop;
 
 	dev = dev_get_by_index(sock_net(sk), so->ifindex);
 	if (!dev) {
 		err = -ENXIO;
-		goto err_out;
+		goto err_out_drop;
 	}
 
 	skb = sock_alloc_send_skb(sk, so->ll.mtu + sizeof(struct can_skb_priv),
 				  msg->msg_flags & MSG_DONTWAIT, &err);
 	if (!skb) {
 		dev_put(dev);
-		goto err_out;
+		goto err_out_drop;
 	}
 
 	can_skb_reserve(skb);
@@ -976,7 +976,7 @@ static int isotp_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
 	if (err) {
 		pr_notice_once("can-isotp: %s: can_send_ret %pe\n",
 			       __func__, ERR_PTR(err));
-		goto err_out;
+		goto err_out_drop;
 	}
 
 	if (wait_tx_done) {
@@ -989,6 +989,9 @@ static int isotp_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
 
 	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)