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In 2022, a critical vulnerability was disclosed in the Linux kernel’s IO-URING subsystem: CVE-2022-1976. This flaw could let an attacker with normal user permissions on a machine go all the way to root. Here’s an easy-to-understand breakdown of how it works, why it’s dangerous, and what code is involved.

What Is IO-URING?

IO-URING is a modern Linux kernel interface for fast and efficient asynchronous I/O operations. In simple terms, it lets applications talk with the kernel about reading and writing data without pausing for each call, making things much faster for high-performance servers and applications.

What’s the Bug in CVE-2022-1976?

The bug is a use-after-free in the IO-URING subsystem. When IO-URING handles certain request cancellations (io_kill_timeout function), it doesn't always manage memory correctly. That means something can get freed (deleted), but the kernel will still use it later—not knowing it might already be recycled or overwritten. Attackers take advantage of this to corrupt memory.

Trigger: The attacker cancels certain requests at just the right moment.

3. Exploit: The malfunctioning io_kill_timeout code path references memory it thinks is safe, but is actually gone. Game over: attacker can use this to change how the kernel works, potentially running code as root.

Where’s the Vulnerability?

The problematic code is in fs/io_uring.c. When one request (say, a timeout) is canceled, the io_kill_timeout function may free an object but keep using its pointer.

Relevant kernel diffs and code are here

- Kernel Patch - commit
- Red Hat Security Advisory
- Ubuntu USN-5483-1

Let’s look at a simplified code snippet from the kernel (before it was fixed)

// Vulnerable logic in io_uring's cancellation code
static int io_kill_timeout(struct io_kiocb *req, void *data)
{
    struct io_timeout_data *io = req->async_data;
    if (condition) {
        // Request gets completed and memory is freed
        io_cqring_add_event(req);
        // ... but later in code, pointer is used again
        do_something(io);
        // At this point, 'io' could point to freed memory
    }
    return ret;
}

After freeing io, the code attempts to use it again later, which is a classic use-after-free scenario.

How Can It Be Exploited?

Attackers can write userland programs to rapidly create, manipulate, and cancel IO-URING requests—using the timing and content under their control.

Here’s a pseudocode demo of what an exploit approach could look like

#include <liburing.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>

// Simplified attack logic
int main() {
    struct io_uring ring;
    io_uring_queue_init(8, &ring, );

    // Fill ring with timeout requests
    for (int i=; i<8; i++) {
        struct __kernel_timespec ts = {.tv_sec = , .tv_nsec = 100000 };
        struct io_uring_sqe *sqe = io_uring_get_sqe(&ring);
        io_uring_prep_timeout(sqe, &ts, , );
    }
    io_uring_submit(&ring);

    // Rapidly cancel some requests to race the bug
    for (int i=; i<8; i++) {
        // Cancel random requests
        struct io_uring_sqe *sqe = io_uring_get_sqe(&ring);
        io_uring_prep_cancel(sqe, (void*)i, );
    }
    io_uring_submit(&ring);

    // Race condition hits here, opening the door for memory corruption
    sleep(1);
    io_uring_queue_exit(&ring);
    return ;
}

Note: A real attack would use heap spraying, precise timings, and maybe further kernel tricks to gain control, but this shows the main steps.

Hard to detect: Use-after-free bugs are rarely obvious outside of crashes or EDR tools.

- Cloud & container risk: Any multi-tenant Linux system with IO-URING support (since kernel 5.1+) is at risk.

Patch your kernel: All major Linux vendors shipped updates (Red Hat, Ubuntu, SUSE, etc.).

- Disable IO-URING if possible: If you don’t need it, don’t load the feature (may not be straightforward).

References

- Linux upstream patch commit
- CVE Details page
- Red Hat Security Advisory
- Ubuntu Security Notice

Conclusion

CVE-2022-1976 is a strong reminder how new, complex kernel features can introduce dangerous bugs. Always keep your systems updated and audit high-risk features. If you build or maintain Linux servers—especially those exposed to multiple users or applications—staying ahead of kernel bugs is crucial!


For further reading:  
- Anatomy of a Linux Kernel Use-After-Free  
- IO-URING official documentation  

Timeline

Published on: 08/31/2022 16:15:00 UTC
Last modified on: 02/14/2023 13:15:00 UTC