can: sun4i_can: populate ndo_change_mtu() to prevent buffer overflow

[ Upstream commit 61da0bd4102c459823fbe6b8b43b01fb6ace4a22 ]

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, sun4ican_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 this line:

	dlc = cf->len;

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 a
couple line below when doing:

	for (i = 0; i < dlc; i++)
		writel(cf->data[i], priv->base + (dreg + i * 4));

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.

Fixes: 0738eff1 ("can: Allwinner A10/A20 CAN Controller support - Kernel module")
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20250918-can-fix-mtu-v1-3-0d1cada9393b@kernel.org


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>