can: isotp: fix potential CAN frame reception race in isotp_rcv()

When receiving a CAN frame the current code logic does not consider
concurrently receiving processes which do not show up in real world
usage.

Ziyang Xuan writes:

The following syz problem is one of the scenarios. so->rx.len is
changed by isotp_rcv_ff() during isotp_rcv_cf(), so->rx.len equals
0 before alloc_skb() and equals 4096 after alloc_skb(). That will
trigger skb_over_panic() in skb_put().

=======================================================
CPU: 1 PID: 19 Comm: ksoftirqd/1 Not tainted 5.16.0-rc8-syzkaller #0
RIP: 0010:skb_panic+0x16c/0x16e net/core/skbuff.c:113
Call Trace:
 <TASK>
 skb_over_panic net/core/skbuff.c:118 [inline]
 skb_put.cold+0x24/0x24 net/core/skbuff.c:1990
 isotp_rcv_cf net/can/isotp.c:570 [inline]
 isotp_rcv+0xa38/0x1e30 net/can/isotp.c:668
 deliver net/can/af_can.c:574 [inline]
 can_rcv_filter+0x445/0x8d0 net/can/af_can.c:635
 can_receive+0x31d/0x580 net/can/af_can.c:665
 can_rcv+0x120/0x1c0 net/can/af_can.c:696
 __netif_receive_skb_one_core+0x114/0x180 net/core/dev.c:5465
 __netif_receive_skb+0x24/0x1b0 net/core/dev.c:5579

Therefore we make sure the state changes and data structures stay
consistent at CAN frame reception time by adding a spin_lock in
isotp_rcv(). This fixes the issue reported by syzkaller but does not
affect real world operation.

Fixes: e057dd3f ("can: add ISO 15765-2:2016 transport protocol")
Link: https://lore.kernel.org/linux-can/d7e69278-d741-c706-65e1-e87623d9a8e8@huawei.com/T/
Link: https://lore.kernel.org/all/20220208200026.13783-1-socketcan@hartkopp.net
Cc: stable@vger.kernel.org
Reported-by: syzbot+4c63f36709a642f801c5@syzkaller.appspotmail.com
Reported-by: NZiyang Xuan <william.xuanziyang@huawei.com>
Signed-off-by: NOliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>

net/can/isotp.c

@@ -56,6 +56,7 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/interrupt.h>
+#include <linux/spinlock.h>
 #include <linux/hrtimer.h>
 #include <linux/wait.h>
 #include <linux/uio.h>
@@ -145,6 +146,7 @@ struct isotp_sock {
 	struct tpcon rx, tx;
 	struct list_head notifier;
 	wait_queue_head_t wait;
+	spinlock_t rx_lock; /* protect single thread state machine */
 };
 
 static LIST_HEAD(isotp_notifier_list);
@@ -615,11 +617,17 @@ static void isotp_rcv(struct sk_buff *skb, void *data)
 
 	n_pci_type = cf->data[ae] & 0xF0;
 
+	/* Make sure the state changes and data structures stay consistent at
+	 * CAN frame reception time. This locking is not needed in real world
+	 * use cases but the inconsistency can be triggered with syzkaller.
+	 */
+	spin_lock(&so->rx_lock);
+
 	if (so->opt.flags & CAN_ISOTP_HALF_DUPLEX) {
 		/* check rx/tx path half duplex expectations */
 		if ((so->tx.state != ISOTP_IDLE && n_pci_type != N_PCI_FC) ||
 		    (so->rx.state != ISOTP_IDLE && n_pci_type == N_PCI_FC))
-			return;
+			goto out_unlock;
 	}
 
 	switch (n_pci_type) {
@@ -668,6 +676,9 @@ static void isotp_rcv(struct sk_buff *skb, void *data)
 		isotp_rcv_cf(sk, cf, ae, skb);
 		break;
 	}
+
+out_unlock:
+	spin_unlock(&so->rx_lock);
 }
 
 static void isotp_fill_dataframe(struct canfd_frame *cf, struct isotp_sock *so,
@@ -1444,6 +1455,7 @@ static int isotp_init(struct sock *sk)
 	so->txtimer.function = isotp_tx_timer_handler;
 
 	init_waitqueue_head(&so->wait);
+	spin_lock_init(&so->rx_lock);
 
 	spin_lock(&isotp_notifier_lock);
 	list_add_tail(&so->notifier, &isotp_notifier_list);