CVE-2025-39986 Affecting linux-6.1 package, versions <6.1.158-1~deb11u1

Note: Versions mentioned in the description apply only to the upstream linux-6.1 package and not the linux-6.1 package as distributed by Debian. See How to fix? for Debian:11 relevant fixed versions and status.

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

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.

• https://security-tracker.debian.org/tracker/CVE-2025-39986
• https://git.kernel.org/stable/c/063539db42203b29d5aa2adf0cae3d68c646a6b6
• https://git.kernel.org/stable/c/2e423e1990f3972cbea779883fef52c2f2acb858
• https://git.kernel.org/stable/c/4f382cc887adca8478b9d3e6b81aa6698a95fff4
• https://git.kernel.org/stable/c/60463a1c138900494cb3adae41142a11cd8feb3c
• https://git.kernel.org/stable/c/61da0bd4102c459823fbe6b8b43b01fb6ace4a22
• https://git.kernel.org/stable/c/7f7b21026a6febdb749f6f6f950427245aa86cce
• https://git.kernel.org/stable/c/a61ff7ac93270d20ca426c027d6d01c8ac8e904c
• https://git.kernel.org/stable/c/de77841652e57afbc46e9e1dbf51ee364fc008e1