can: mcba_usb: populate ndo_change_mtu() to prevent buffer overflow

CVE-2025-39985

What is CVE-2025-39985?

In the Linux kernel, the following vulnerability has been resolved:

can: mcba_usb: populate ndo_change_mtu() to prevent buffer overflow

Sending an PF_PACKET allows to bypass the CAN framework logic and to directly reach the xmit() function of a CAN driver. The only check which is performed by the PF_PACKET framework is to make sure that skb->len fits the interface's MTU.

Unfortunately, because the mcba_usb driver does not populate its net_device_ops->ndo_change_mtu(), it is possible for an attacker to configure an invalid MTU by doing, for example:

$ ip link set can0 mtu 9999

After doing so, the attacker could open a PF_PACKET socket using the ETH_P_CANXL protocol:

socket(PF_PACKET, SOCK_RAW, htons(ETH_P_CANXL))

to inject a malicious CAN XL frames. For example:

struct canxl_frame frame = {
	.flags = 0xff,
	.len = 2048,
};

The CAN drivers' xmit() function are calling can_dev_dropped_skb() to check that the skb is valid, unfortunately under above conditions, the malicious packet is able to go through can_dev_dropped_skb() checks:

1. the skb->protocol is set to ETH_P_CANXL which is valid (the function does not check the actual device capabilities).

2. the length is a valid CAN XL length.

And so, mcba_usb_start_xmit() receives a CAN XL frame which it is not able to correctly handle and will thus misinterpret it as a CAN frame.

This can result in a buffer overflow. The driver will consume cf->len as-is with no further checks on these lines:

usb_msg.dlc = cf->len;

memcpy(usb_msg.data, cf->data, usb_msg.dlc);

Here, cf->len corresponds to the flags field of the CAN XL frame. In our previous example, we set canxl_frame->flags to 0xff. Because the maximum expected length is 8, a buffer overflow of 247 bytes occurs!

Populate net_device_ops->ndo_change_mtu() to ensure that the interface's MTU can not be set to anything bigger than CAN_MTU. By fixing the root cause, this prevents the buffer overflow.

References