CVE-2019-25162 - Resolving I2C Vulnerability in Linux Kernel

Linux kernel serves as the foundation of numerous operating systems, including many popular distributions such as Ubuntu, Fedora, and Debian. In the past, researchers have discovered multiple vulnerabilities that could threaten the system's integrity, leading to exploits like unauthorized access and malware execution. One such vulnerability that emerged in the Linux kernel is related to the Inter-Integrated Circuit (I2C) subsystem, commonly used to establish communication between electronic devices. This issue was assigned the Common Vulnerabilities and Exposure identifier CVE-2019-25162.

The I2C Vulnerability

CVE-2019-25162 was detailed as a potential use after free scenario in the i2c subsystem of the Linux kernel. The 'use after free' vulnerability occurs when a memory block is continued to be accessed after it has been deallocated. This can lead to issues like memory corruption, incorrect execution of program code, and, in the worst case, arbitrary code execution by malicious exploiters.

The vulnerability was spotted in the i2c-core-base.c file, specifically in the i2c_unregister_device() function. Researchers found that the issue emerged during the freeing of an 'adap' structure, which is only meant to be deallocated after it is done being used. However, the use of put_device() function in the original code was causing the 'adap' structure to be freed prematurely, creating a use after free vulnerability.

Fixing CVE-2019-25162

To patch this vulnerability, the code in the i2c subsystem was altered to move the put_device() function call further down, ensuring that the 'adap' structure is not freed before its intended time.

The original code snippet with the vulnerability

/* unregister device from i2c bus */
static int i2c_unregister_device(struct device *dev, void *null)
{
    struct i2c_client *client = to_i2c_client(dev);
    struct i2c_adapter *adap = client->adapter;

    ... // other variable declarations

    put_device(&client->dev);

    // i2c_unregister_device will unregister the device and call
    // the remove() method on the attached driver (if any).
    i2c_unregister_device(client);

    ...
}

The fixed code snippet after patching the vulnerability

/* unregister device from i2c bus */
static int i2c_unregister_device(struct device *dev, void *null)
{
    struct i2c_client *client = to_i2c_client(dev);
    struct i2c_adapter *adap = client->adapter;

    ... // other variable declarations

    // i2c_unregister_device will unregister the device and call
    // the remove() method on the attached driver (if any).
    i2c_unregister_device(client);

    put_device(&client->dev); // moved down to avoid use after free

    ...
}

As the code snippet above demonstrates, the put_device() function was simply moved down a few lines to prevent the 'adap' structure from being deallocated before it had been utilized completely, thus fixing the vulnerability.

Original References

1. Linux Kernel Git Repository - Fix i2c vulnerability
2. CVE-2019-25162 - National Vulnerability Database

Conclusion

In conclusion, CVE-2019-25162 represented a potential use after free vulnerability in the i2c subsystem of the Linux kernel. By examining and altering the code, developers were able to remedy the issue by moving the put_device() function call further down, preventing the premature deallocation of the 'adap' structure. This fix not only has patched the specific vulnerability but also serves as a demonstration of the continuous effort to maintain and improve the security of the popular Linux kernel.

Timeline

Published on: 02/26/2024 18:15:07 UTC
Last modified on: 04/17/2024 17:38:07 UTC