Understanding the CVE-2021-46985 fix in the Linux kernel - Resolving memory leaks in ACPI scan error handling

In the Linux kernel, a recent vulnerability identified as CVE-2021-46985 caught the attention of the developer community. This vulnerability involved a memory leak that could occur in the kernel's ACPI scan subsystem while handling errors. The fix for this issue ensures that the proper resource clean-up is performed when errors are encountered in the ACPI device naming process.

For those unfamiliar with the Linux kernel and ACPI, understanding this vulnerability and its fix may be slightly tricky. This post aims to decode the vulnerability, the patch, and the code snippet that resolves the issue using simple American English.

What is ACPI?

The Advanced Configuration and Power Interface (ACPI) is a standard developed by several major computer hardware companies for OS-directed configuration and power management. In operating systems such as Linux, the ACPI protocol helps manage resources, power savings, and other aspects of computer hardware.

The vulnerability

The CVE-2021-46985 vulnerability relates to the ACPI scanning process in the Linux kernel. To better understand this, let's break it down into smaller components.

In the kernel's ACPI: scan subsystem, there's a function called 'acpi_device_set_name()' responsible for setting the device name property of an ACPI device object. In certain cases, this function can fail, meaning that it encounters an error. When this happens, developers must ensure that all allocated resources are properly cleaned up to prevent memory leaks.

A memory leak is a situation where allocated memory isn't released when it should be, causing the program to consume increasing amounts of memory and potentially leading to performance-related issues or even system crashes.

The fix

To fix the memory leak vulnerability, the kernel developers needed to ensure that the 'acpi_device_bus_id->bus_id' resource gets freed whenever the 'acpi_device_set_name()' function encounters an error. By doing so, they made certain that memory allocation and clean-up processes are correctly managed in error scenarios.

Here's the code snippet that shows the implementation of this fix

diff --git a/drivers/acpi/scan.c b/drivers/acpi/scan.c
index ef27cbc..fdb44d 100644
--- a/drivers/acpi/scan.c
+++ b/drivers/acpi/scan.c
@@ -294,6 +294,7 @@ static int acpi_add_single_object(struct acpi_device **child,
    return 1;
 
err:
+   kfree(acpi_device_bus_id->bus_id);
    return result;
 }

The patch adds a simple line of code to the error handling part of the function, ensuring that 'acpi_device_bus_id->bus_id' is freed whenever the 'acpi_device_set_name()' function fails.

Original references

For those interested in learning more about this vulnerability and its fix, the following resources offer additional information:

1. Official commit message: ACPI: scan: Fix a memory leak in an error handling path
2. Link to the full patch containing the code implementation: Patch on kernel.org

Exploit details

At the time of writing, there are no known exploits available that abuse this memory leak vulnerability in the wild. However, the deployed fix works to ensure that the Linux kernel is more robust when handling errors in ACPI scan processes, reducing the risk of any associated impact due to potential memory leaks.

Timeline

Published on: 02/28/2024 09:15:37 UTC
Last modified on: 05/29/2024 05:00:01 UTC