A new vulnerability, CVE-2024-27039, was discovered and patched in the Linux kernel, specifically affecting the HiSilicon hi3559a clock driver. This post will explain what went wrong in the code, how the vulnerability happened, its potential impact, and show details including the exploit area—using simple, clear language.

The Vulnerability at a Glance

This bug affects the clk (clock) driver for HiSilicon SoCs (system-on-chips), commonly used in networking or multimedia devices.

The vulnerability lies in how memory is freed (kfree) in the driver.

- The issue is a classic pointer bug: the code tries to free memory using a pointer (p_clk) that could have been incremented away from the original memory block due to a failed loop in the registration process.
- If this memory is freed, but the pointer is incorrect, it can cause memory corruption, crashes, or undefined behavior—potentially opening the door for privilege escalation or further exploitation.

*Simplified Explanation*

- An array of pointers named p_clk is allocated to keep track of fixed clocks that need registration.

On failure, it cleans up by calling devm_kfree() on p_clk.

- The problem? p_clk is changed (incremented) inside the loop, so on failure, it may no longer point to the start of the array that was originally allocated.

Vulnerable Code Snippet

p_clk = devm_kzalloc(dev, num_clks * sizeof(*p_clk), GFP_KERNEL);
for (i = ; i < num_clks; i++, p_clk++) {
    *p_clk = clk_register(...)
    if (IS_ERR(*p_clk))
        goto err_register;
}
...
err_register:
    devm_kfree(dev, p_clk); // p_clk is not the original pointer!

What’s wrong?

By the time an error is hit, p_clk isn’t at the start of the array anymore, since the loop did p_clk++. Freeing memory like this is dangerous and can corrupt the kernel’s memory.

The Fix

The patch simply removes the wrong devm_kfree() call to avoid improper memory release.

Corrected Code Snippet

// err_register:
//     devm_kfree(dev, p_clk); // <-- This line is now removed/fixed

This way, it doesn’t free the wrong address, and the memory is safely managed by the device memory model (devm_ functions).


## Exploit/Impact Details

While this bug doesn’t have a public “exploit” in the usual sense, here's what could go wrong in theory:

- Malicious user or code could trigger the driver to load and fail during clock registration (possible by manipulating hardware state or device tree data).
- The kernel would then free memory at a wrong location, corrupting the memory allocator’s linked list.
- On some kernels or with certain configurations, attackers might use this to escalate privileges, cause denial-of-service, or crash the system.

Since this is a logic error, a simple test case could be

// This is conceptual, not an "exploit" per se
echo "fake-device-node" > /sys/bus/platform/devices/trigger_hi3559a;
# This triggers device probing and potential registration failure.
# With the bug, kernel memory can get corrupted here.

But practically, tailored device trees or hardware would be needed for true exploitation.

References

- Upstream Patch (LKML)
- NVD CVE-2024-27039
- Linux Kernel Source (drivers/clk/hisilicon/clk-hi3559a.c)

How to Stay Safe

- Update your kernel if you run HiSilicon chips, especially if using custom hardware in routers, DVRs, etc.
- Watch your kernel logs for weird crashes if using hi3559a—panic logs mentioning devm_kfree could be a clue.

In Summary

CVE-2024-27039 shows how a single pointer increment bug in a device driver can open up serious issues in the kernel. It’s a great reminder: always be careful with pointers, especially when freeing memory inside loops.

Stay safe, keep your kernels patched, and subscribe for more simple, deep dives into Linux security!

Timeline

Published on: 05/01/2024 13:15:49 UTC
Last modified on: 09/18/2025 15:48:40 UTC