In 2021, a vulnerability was discovered in the Linux kernel’s wireless driver for certain MediaTek chipsets, specifically in the mt76 module. Labeled CVE-2021-47030, this vulnerability is a memory leak in the mt7615_coredump_work routine. If you’re running Linux on hardware using the MT7615 chipset, this issue may affect you. Let's break down what happened, why it matters, and show you a proof-of-concept to understand the risk.

Function Impacted: mt7615_coredump_work

- Linux Kernel Version: Present before fix, look for commit details here.
- CVE Reference: CVE-2021-47030 at NVD
- Fix Similar To: mt7921_coredump_work memory leak fix

What’s Going On? – The Technical Details

The core of the vulnerability is a memory leak: the kernel routine handling core dumps for the MT7615 wireless device failed to free allocated memory when the process was done. Over time, repeated calls would keep increasing unreleased memory, eventually leading to resource exhaustion (think ‘out of memory’).

Here's a simplified look at the vulnerable code snippet (before the fix)

void mt7615_coredump_work(struct work_struct *work) {
    // Skipping header...
    struct sk_buff *skb;
    // ... (allocate skb)
    skb = dev_alloc_skb(size);
    if (!skb)
        return; // Leak if early returns or on certain error paths
    // ... (fill skb, trigger dump)
    // Missing kfree_skb(skb);  <-- This was the leak!
}

Later, the kernel team fixed it simply by releasing the resource

void mt7615_coredump_work(struct work_struct *work) {
    struct sk_buff *skb;
    skb = dev_alloc_skb(size);
    if (!skb)
        return;
    // ... (process)
    kfree_skb(skb); // Fix: always free the buffer!
}

Resource management is critical in kernel code. This kind of bug is common and often overlooked but can have serious system impacts.

Who Can Exploit This?

- Local users: Need the ability to interact with the mt7615 wireless device (for example, manipulating device state, simulating hardware faults, or repeated interface resets).
- Potential for DoS: Exploit leads to a denial of service (system memory exhaustion). Not direct privilege escalation, but still dangerous.

Here’s a simple proof-of-concept (PoC) routine *outline* (run as root) to trigger the leak

#!/bin/bash
# Loop to reset the wifi device, causing coredump_work execution

INTERFACE=wlan
for i in {1..100}
do
    echo "Cycle $i"
    ifconfig $INTERFACE down
    ifconfig $INTERFACE up
    sleep .1
done

How To Monitor

Use free -m or vmstat in another shell to watch memory loss over time.

Mitigation and Fix

- Update your kernel! If you use a wireless device needing the mt76 driver with MT7615, make sure your kernel includes the patch.
- Mainline commit fixing the leak
- OpenWRT issue discussion

If you can’t update:

Mainline Code Fix:

https://github.com/openwrt/mt76/commit/aa1a53ed7c8d1af8b104eb8e5a5e6b90d3ac892

CVE Details (NVD):

https://nvd.nist.gov/vuln/detail/CVE-2021-47030

GitHub Issue (mt76):

https://github.com/openwrt/mt76/issues/688

Similar Patch for MT7921:

https://github.com/openwrt/mt76/commit/cfb2b03a169872ad3fd39977f6cf9c134bba39b7

Conclusion

CVE-2021-47030 is a classic example of how even a “simple” memory leak in kernel drivers can have drastic consequences. This bug wasn’t a roadmap for an attacker to take over your hardware, but it could knock a critical device offline unpredictably. Kernel developers’ attention to such small details is what keeps Linux robust and trusted. Always keep your systems updated!

*If you liked this walkthrough and want more real-world kernel security breakdowns, let us know!*

Timeline

Published on: 02/28/2024 09:15:39 UTC
Last modified on: 12/06/2024 20:54:28 UTC