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The security of the Linux kernel is crucial, especially when it comes to networking components. In early 2022, a vulnerability was found in the widely-used Marvell DSA (Distributed Switch Architecture) driver (mv88e6xxx), tracked as CVE-2022-49367. This issue could lead to resource exhaustion and potential denial-of-service conditions due to a reference count leak in the kernel’s device management functions.

In this exclusive deep dive, we’ll explain how the bug was introduced, how it was fixed, and provide code samples to understand the risk and remediation. This guide is meant for kernel developers, sysadmins, and anyone curious about how small bugs can have large, lurking impacts.

Component: Linux kernel, Marvell mv88e6xxx driver (part of the DSA subsystem)

- File(s): drivers/net/dsa/mv88e6xxx/mv88e6xxx_main.c, mv88e6xxx_mdios_register()

Impact: Reference (refcount) leak on device tree nodes

- CVE-ID: CVE-2022-49367

The Vulnerability

The bug was due to improper management of device node references in the kernel’s device-tree handling. When calling of_get_child_by_name(), the kernel increments a reference counter for safety. However, the driver failed to release taht reference with of_node_put() once finished, resulting in a leak.

Why It Matters

Unreleased references accumulate in the kernel, preventing the freeing of memory or device structures. With enough leaks or over extended uptimes, it could cause resource exhaustion or even a system crash.

Let’s look at the offending code for context. (Note: simplified for clarity)

static int mv88e6xxx_mdios_register(struct mv88e6xxx_chip *chip)
{
    struct device_node *mdio_np;

    mdio_np = of_get_child_by_name(chip->dev->of_node, "mdio");
    if (!mdio_np)
        return -ENODEV;

    // Registers a new MDIO bus
    err = of_mdiobus_register(bus, mdio_np);

    // MISSING: of_node_put(mdio_np);

    return err;
}

of_mdiobus_register(): Registers the device, but does not take over the reference.

- PROBLEM: The driver never calls of_node_put(mdio_np) – leaking a reference each time this function is called.

The Corrected Code

The upstream patch simply adds the necessary release:

static int mv88e6xxx_mdios_register(struct mv88e6xxx_chip *chip)
{
    ...
    mdio_np = of_get_child_by_name(chip->dev->of_node, "mdio");
    if (!mdio_np)
        return -ENODEV;

    err = of_mdiobus_register(bus, mdio_np);

    of_node_put(mdio_np); // CORRECT: Release reference

    return err;
}

Now, every reference grabbed by of_get_child_by_name() will be released with of_node_put(), preventing leaks.

4. How to Exploit (for Understanding)

Note: This is an unprivileged leak, not a classic "remote code execution" bug. Instead, exploitation could look like:

- A local user, or a system component, repeatedly triggers the DSA MDIO register code (e.g., by hotplug events).

Theoretically, to reproduce the leak (requires root permissions)

# Repeatedly reload the mv88e6xxx driver (simulate switch hotplug events)
for i in $(seq 1 100); do
    rmmod mv88e6xxx; modprobe mv88e6xxx
done

# dmesg may show errors, system may become unstable after enough cycles

WARNING: Only attempt this in test environments, not on production!

5. Mitigation & Detection

- Mitigation: Apply the kernel patch. All major distributions (as of 2023) ship the fix. Check your kernel’s build date and commit log.

`sh

cat /proc/device-tree | grep -c ''
<br>- <b>Audit:</b> Check dmesg` for refcount warnings or OOM errors related to device management.

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## 6. References

- CVE-2022-49367 NVD entry
- Upstream Kernel Patch
- DSA Marvell driver documentation
- of_get_child_by_name() Kernel API

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## 7. Summary

CVE-2022-49367 looks simple: just a missing line of code! But in systems programming, tiny details can topple giants. If you or your infrastructure relies on Marvell-based Linux networking switches, ensure your kernel is up-to-date. Review critical upstream patches regularly—and remember, *a single reference leak can take down an enterprise system.* Stay patched and vigilant!

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Timeline

Published on: 02/26/2025 07:01:13 UTC
Last modified on: 04/14/2025 20:42:13 UTC