Subject: [RFC PATCH v3] can: isotp: fix CAN frame reception race in isotp_rcv()

From: Oliver Hartkopp <[email-masked]>
Date: Fri, 28 Jan 2022 09:20:46 +0100

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.

Link: https://lore.kernel.org/linux-can/[email-masked]/T/
Fixes: e057dd3fc20f ("can: add ISO 15765-2:2016 transport protocol")
Reported-by: [email-masked]
Reported-by: Ziyang Xuan <[email-masked]>
Signed-off-by: Oliver Hartkopp <[email-masked]>
---
 net/can/isotp.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/net/can/isotp.c b/net/can/isotp.c
index 02cbcb2ecf0d..bd281fdbe855 100644
--- a/net/can/isotp.c
+++ b/net/can/isotp.c
@@ -54,10 +54,11 @@
  */
 
 #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>
 #include <linux/net.h>
 #include <linux/netdevice.h>
@@ -143,10 +144,11 @@ struct isotp_sock {
 	u32 force_tx_stmin;
 	u32 force_rx_stmin;
 	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);
 static DEFINE_SPINLOCK(isotp_notifier_lock);
 static struct isotp_sock *isotp_busy_notifier;
@@ -613,15 +615,24 @@ static void isotp_rcv(struct sk_buff *skb, void *data)
 	if (ae && cf->data[0] != so->opt.rx_ext_address)
 		return;
 
 	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.
+	 *
+	 * To not lock up the softirq just drop the frame in syzcaller case.
+	 */
+	if (!spin_trylock(&so->rx_lock))
+		return;
+
 	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) {
 	case N_PCI_FC:
 		/* tx path: flow control frame containing the FC parameters */
@@ -666,10 +677,13 @@ static void isotp_rcv(struct sk_buff *skb, void *data)
 	case N_PCI_CF:
 		/* rx path: consecutive frame */
 		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,
 				 int ae, int off)
 {
@@ -1442,10 +1456,11 @@ static int isotp_init(struct sock *sk)
 	so->rxtimer.function = isotp_rx_timer_handler;
 	hrtimer_init(&so->txtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_SOFT);
 	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);
 	spin_unlock(&isotp_notifier_lock);
 
-- 
2.30.2