Bug 2404103 (CVE-2025-39987)

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

can: hi311x: 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 sun4i_can 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, hi3110_hard_start_xmit() receives a CAN XL frame which it is
not able to correctly handle and will thus misinterpret it as a CAN
frame. The driver will consume frame->len as-is with no further
checks.

This can result in a buffer overflow later on in hi3110_hw_tx() on
this line:

	memcpy(buf + HI3110_FIFO_EXT_DATA_OFF,
	       frame->data, frame->len);

Here, frame->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.