In 2022, a serious vulnerability was discovered in the Linux kernel: CVE-2022-1998. This security flaw is a use-after-free bug found in the kernel's file system notify (fsnotify) subsystem. By carefully triggering this bug, a local attacker could crash the system or even gain higher privileges. In this post, we'll break down what happened, how it was fixed, and how it could be exploited in the real world. We'll also show simple code snippets to help you understand the vulnerability.

The Basics: What Is a Use-After-Free?

First, let's clarify what a "use-after-free" bug is:
A use-after-free happens when a program frees (releases) a chunk of memory, but then continues to use it. Because the memory is no longer valid, this can let attackers do unexpected things like crashing the program, corrupting data, or even running their own code.

Next function in line: copy_info_records_to_user()

These functions handle sending file system event notifications from the kernel to user-space programs. This is fundamental for things like inotify and fanotify.

If the second function (copy_info_records_to_user()) encounters a problem, it fails.

4. Unfortunately, if that happens, kernel memory related to the event gets freed, but another part of the kernel still tries to use it afterward.

This is a classic use-after-free: memory is freed but used again.

Key Code Snippet (Simplified)

Below, we've built a *simplified* snippet to show the problem. This is not the *exact* code but a representation:

// fs/notify/fanotify/fanotify_user.c

struct fanotify_event_info {
    struct some_info *info;
    // ... other members
};

int copy_event_to_user(struct fanotify_event_info *event, void __user *buf) {
    int ret = copy_info_records_to_user(event->info, buf);
    if (ret < ) {
        kfree(event->info);  // Freed memory
        // ... OOPS: Later code still uses event->info!
    }
    // For example:
    if (event->info->additional_field) { // use-after-free!
        // ...do something...
    }
    return ret;
}

After freeing event->info, continuing to use it leads to unpredictable results—this is where the danger comes in.

Impact: Why Does It Matter?

Any local user with permission to use inotify/fanotify (usually most users on a desktop/server) can trigger this bug. The two main risks are:

Trigger a notification event (like opening or changing the file).

3. Manipulate user-space buffers to make copy_info_records_to_user() fail (for instance, by interrupting the copy to user memory).

Cause use-after-free logic in the kernel, leading to memory corruption.

5. With kernel memory corrupted, they can try to overwrite sensitive data structures—potentially running code as root.

Note: Kernel exploitation is complex and not always guaranteed, but this bug offered a clear path for attackers skilled in Linux kernel internals.

Example Exploit Outline (Pseudocode)

// This is a simplified logic flow, not a full exploit!
int fd = fanotify_init(...);
// Add a malicious watch
fanotify_mark(fd, FAN_MARK_ADD, ..., ...);
// Trigger event (e.g., modify the file)
// Cause copy to user to fail (e.g., manipulate buffer pointers)
// Observe system crash or attempt controlled overwrite for privilege escalation

How Was It Fixed?

The kernel patch for CVE-2022-1998 ensures that after freeing memory, no part of the code uses the freed memory again. It reorganizes the logic in the affected functions and adds proper checks after freeing resources.

Patch Example

if (ret < ) {
    kfree(event->info);
    event->info = NULL;  // Prevent further usage
    return ret;
}

By nullifying the pointer and returning immediately, later code doesn't risk using released memory.

References and Further Reading

- Red Hat CVE-2022-1998 Bugzilla Entry
- NVD Details for CVE-2022-1998
- Kernel Patch (LWN.net)

Conclusion

CVE-2022-1998 is a great example of how a subtle bug in low-level code can have serious consequences for system security and stability. Use-after-free bugs are particularly nasty, especially in the kernel, where they might allow local attackers to crash machines or even become root.

If you manage Linux systems, make sure you're running an up-to-date kernel with this patch applied. Always be cautious with what permissions you allow local users; even "non-root" users can escalate privileges through such flaws.


Stay safe and patch your systems!
If you have deeper questions about CVE-2022-1998, feel free to comment below or check out the official references.

Timeline

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