CVE-2025-39985

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

can: mcbausb: populate ndochange_mtu() to prevent buffer overflow

Sending an PFPACKET 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 PFPACKET framework is to make sure that skb->len fits the interface's MTU.

Unfortunately, because the mcbausb driver does not populate its netdeviceops->ndochange_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 PFPACKET socket using the ETHP_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 candevdroppedskb() to check that the skb is valid, unfortunately under above conditions, the malicious packet is able to go through candevdroppedskb() checks:

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

2. the length is a valid CAN XL length.

And so, mcbausbstart_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 netdeviceops->ndochangemtu() 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.