linux-kernel - [PATCH 3/4] can: sun4i_can: populate ndo_change

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Message-Id: <20250918-can-fix-mtu-v1-3-0d1cada9393b@kernel.org>
Date: Thu, 18 Sep 2025 18:00:26 +0900
From: Vincent Mailhol <mailhol@...nel.org>
To: Marc Kleine-Budde <mkl@...gutronix.de>, 
 Oliver Hartkopp <socketcan@...tkopp.net>
Cc: Arunachalam Santhanam <arunachalam.santhanam@...bosch.com>, 
 Akshay Bhat <akshay.bhat@...esys.com>, 
 Wolfgang Grandegger <wg@...ndegger.com>, Chen-Yu Tsai <wens@...e.org>, 
 Jernej Skrabec <jernej.skrabec@...il.com>, 
 Samuel Holland <samuel@...lland.org>, Maxime Ripard <mripard@...nel.org>, 
 Gerhard Bertelsmann <info@...hard-bertelsmann.de>, 
 Yasushi SHOJI <yashi@...cecubics.com>, 
 Remigiusz Kołłątaj <remigiusz.kollataj@...ica.com>, 
 linux-can@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev, 
 Vincent Mailhol <mailhol@...nel.org>
Subject: [PATCH 3/4] can: sun4i_can: 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, 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: 0738eff14d81 ("can: Allwinner A10/A20 CAN Controller support - Kernel module")
Signed-off-by: Vincent Mailhol <mailhol@...nel.org>
---
 drivers/net/can/sun4i_can.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/can/sun4i_can.c b/drivers/net/can/sun4i_can.c
index 6fcb301ef611d0c8bdb8720aaa77dc78950123d6..53bfd873de9bdecaf6923049007f9efd71289dd3 100644
--- a/drivers/net/can/sun4i_can.c
+++ b/drivers/net/can/sun4i_can.c
@@ -768,6 +768,7 @@ static const struct net_device_ops sun4ican_netdev_ops = {
 	.ndo_open = sun4ican_open,
 	.ndo_stop = sun4ican_close,
 	.ndo_start_xmit = sun4ican_start_xmit,
+	.ndo_change_mtu = can_change_mtu,
 };

 static const struct ethtool_ops sun4ican_ethtool_ops = {

-- 
2.49.1