[PATCH] bonding: refuse to enslave CAN devices

[Oliver Hartkopp]

syzbot reported a kernel paging request crash in
can_rx_unregister() inside net/can/af_can.c. The crash occurs
because a virtual CAN device (vxcan) is being enslaved to a
bonding master.

During the enslavement process, the bonding driver mutates
and modifies the network device states to fit an Ethernet-like
aggregation model. However, CAN devices operate on a completely
different Layer 2 architecture, relying on the CAN mid-layer
private data structure (can_ml_priv) instead of standard
Ethernet structures. Since bonding does not initialize or
maintain these CAN structures, subsequent operations on the
half-enslaved interface (such as closing associated sockets
via isotp_release) lead to a null-pointer dereference when
accessing the CAN receiver lists.

Bonding CAN interfaces is architecturally invalid as CAN lacks
MAC addresses, ARP capabilities, and standard Ethernet
link-layer mechanisms. While generic loopback devices are
blocked globally in net/core/dev.c, virtual CAN devices
bypass this check because they do not carry the IFF_LOOPBACK
flag, despite acting as local software-loopbacks.

Fix this by explicitly blocking network devices of type
ARPHRD_CAN from being enslaved at the very beginning of
bond_enslave(). This prevents illegal state mutations,
eliminates the resulting KASAN crashes, and avoids potential
memory leaks from incomplete socket cleanups.

As the CAN support has been added a long time after bonding
the Fixes-tag points to the introduction of ARPHRD_CAN that
would have needed a specific handling in bonding_main.c.

Fixes: cd05acfe65ed ("[CAN]: Allocate protocol numbers for PF_CAN")
Reported-by: syzbot+8ed98cbd0161632bce95@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=8ed98cbd0161632bce95
Signed-off-by: Oliver Hartkopp <socketcan@xxxxxxxxxxxx>
---
 drivers/net/bonding/bond_main.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index af82a3df2c5d..82e779f7916b 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -1888,10 +1888,16 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
 	const struct net_device_ops *slave_ops = slave_dev->netdev_ops;
 	struct slave *new_slave = NULL, *prev_slave;
 	struct sockaddr_storage ss;
 	int res = 0, i;
 
+	if (slave_dev->type == ARPHRD_CAN) {
+		BOND_NL_ERR(bond_dev, extack,
+			    "CAN devices cannot be enslaved");
+		return -EPERM;
+	}
+
 	if (slave_dev->flags & IFF_MASTER &&
 	    !netif_is_bond_master(slave_dev)) {
 		BOND_NL_ERR(bond_dev, extack,
 			    "Device type (master device) cannot be enslaved");
 		return -EPERM;

---
base-commit: d60ec36cab338dfe2ae40d73e9c8d6c4af70d2b8
change-id: 20260526-bonding-candev-f4a0cf2eee9b

Best regards,
--  
Oliver Hartkopp <socketcan@xxxxxxxxxxxx>