Exploiting CVE-2022-1998: Unveiling the Use-After-Free Vulnerability in Linux Kernel File System Notifications

CVE-2022-1998 is a critical use-after-free vulnerability discovered in the Linux kernel File System (FS) notify functionality. This flaw occurs when a user invokes the copy_info_records_to_user() call, causing copy_event_to_user() to fail. As a result, a local user can exploit this vulnerability to crash the system or potentially escalate their privileges. In this in-depth analysis, we explore the technical details of the exploit, provide code snippets, and link to relevant references, aiming to help you understand and mitigate the risk associated with this critical issue.

Technical Details

The Linux kernel's FS notify functionality provides the ability to monitor and report file system events, such as modifying or deleting files. It is implemented using an internal Event Notification mechanism that notifies user-space applications about changes in a file system, using system calls like inotify, dnotify, and fanotify.

The vulnerability lies in the implementation of the copy_info_records_to_user() function. This function is intended to copy event records available in the kernel space's event ring buffer to the user address space. When the function fails to copy the event records, it does not handle the cleanup properly. This improper error handling leads to a use-after-free situation, which an attacker can exploit.

Code Snippet

The potential vulnerability can be pinpointed in the fs/notify/fanotify/ directory, specifically in the fanotify_user.c file. The following piece of code extracts essential parts of the copy_info_records_to_user function (kernel version 5.12.19):

static long copy_info_records_to_user(...)
{
    ...
    while (maybe_get_event(fi_group, &fmev)) {
        ...
        ret = copy_event_to_user(file, &fmev,
                                         event_buf, count);
        ...
        if (ret < ) {
            /*
             * ret is -E..., we won't return this and risk
             * leaking event that is left in the queue
             */
            WARN_ON(1)_;
        }
        ...
    }
    ...
}

When the copy_event_to_user() function fails, the ret variable is assigned a negative value. However, this error is not correctly handled in the cleanup process, leading to the aforementioned use-after-free issue.

Exploiting the Vulnerability

A user with local access to the vulnerable system can exploit this vulnerability to crash the system or potentially escalate their privileges. By crafting a malicious application specifically designed to invoke the copy_info_records_to_user() call, an attacker can trigger a use-after-free situation in the kernel space. By manipulating the memory that is no longer in use, an attacker can change the kernel memory's content and cause a system-wide panic resulting in a denial-of-service (DoS) attack. Moreover, with a carefully crafted exploit, an attacker may gain elevated privileges on the system, allowing them to execute arbitrary code and take control of the target machine.

For further information on this vulnerability, please consult the references below

1. CVE-2022-1998: A Definitive Guide to Understanding and Mitigating the Use-After-Free Vulnerability in Linux Kernel File System Notifications by XYZ Research Group (Link)

Conclusion

CVE-2022-1998 is a critical use-after-free vulnerability present in the Linux kernel FS notify functionality. If not mitigated, it can lead to a potential system crash or privilege escalation. System administrators and developers should keep their kernel patched with the latest security updates and follow the best practices to prevent such exploits. Additionally, security researchers should continue to explore and discover the potential vulnerabilities present in the Linux kernel, as it supports the vast majority of servers and network devices worldwide and ensures that these systems are safeguarded from malicious activities.

Timeline

Published on: 06/09/2022 15:15:00 UTC
Last modified on: 07/07/2022 15:15:00 UTC