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TL;DR

A critical use-after-free (UAF) bug was found and patched in the Linux kernel’s dm900 network driver, officially tracked as CVE-2025-21715. The bug happened in the dm900_drv_remove function which could accidentally access freed memory, risking kernel crashes or more severe vulnerabilities. Let’s break down what went wrong, why it’s dangerous, and how it got fixed.

What is CVE-2025-21715?

CVE-2025-21715 refers to a vulnerability discovered in the Linux kernel’s DM900 network driver. DM900 is an Ethernet controller often used on embedded systems.

The bug was a use-after-free (UAF) in the dm900_drv_remove function. Basically, the code tried to use a memory structure (dm) after it had already been freed with free_netdev(). This is always bad news in kernel code—when you use memory after it’s freed, the system could behave unpredictably, crash, or, in worst cases, attackers could exploit it for privilege escalation.

Here's the problematic function (simplified for clarity)

static int dm900_drv_remove(struct platform_device *pdev)
{
    struct net_device *ndev = platform_get_drvdata(pdev);
    struct board_info *dm = netdev_priv(ndev);

    unregister_netdev(ndev);
    free_netdev(ndev);
    // dm is now dangling/freed!

    /* Problem: code below still uses dm! */
    if (dm->some_flag)
        do_something(); // unsafe UAF
    return ;
}

- free_netdev(ndev); releases (frees) the memory associated with both ndev and its private dm900 structure (dm).

Exploit Details

If a malicious user or attacker knows how to trigger device removal (for example, by removing a gadget or hot-unplugging a device), they could potentially exploit this bug:

Once the network device is being removed via dm900_drv_remove, memory for dm is freed.

2. But any code using dm after it's freed can read garbage, or write to memory now owned by something else.
3. With advanced exploitation, there’s a possibility to use this flaw to overwrite kernel data, deny service, or attempt privilege escalation, especially if the freed memory is repurposed.

In practice, this would be tricky, but it’s an unacceptable risk, especially for critical embedded or IoT devices.

The Fix: Put Free at the End

To resolve the vulnerability, the call to free_netdev() was moved to the very end of the function—after all uses of the dm pointer.

Here’s the corrected version

static int dm900_drv_remove(struct platform_device *pdev)
{
    struct net_device *ndev = platform_get_drvdata(pdev);
    struct board_info *dm = netdev_priv(ndev);

    unregister_netdev(ndev);

    // Do all operations with 'dm' here, before freeing
    if (dm->some_flag)
        do_something();

    free_netdev(ndev); // Free at very end, after all use!

    return ;
}

This way, there’s no chance of touching freed memory.

Linux kernel fix commit:

net: davicom: fix UAF in dm900_drv_remove

Similar previous bug:

[ad297cd2db89] net: qcom/emac: fix UAF in emac_remove
(commit)

- Official Linux Kernel Source Browser: dm900.c
- Static Analysis Detection Tools

Why This Matters

- Use-after-free bugs are among the most exploited vulnerability types in C/C++ software.
- This vulnerability, while not remotely exploitable in most setups, could be used for targeted attacks on hardware relying on the dm900 chip.

📢 For developers:

Look at how resource deallocation order matters. Don’t trust that freeing is “just” a cleanup detail—it can change your whole security profile.


References:
- CVE-2025-21715 at MITRE (when available)
- Linux Kernel vulnerability list
- Commit: net: davicom: fix UAF in dm900_drv_remove


*Spotted by our static analysis tool—don’t skip those builds!*

Timeline

Published on: 02/27/2025 02:15:15 UTC
Last modified on: 05/04/2025 13:06:26 UTC