A recent vulnerability has been discovered and resolved in the Linux kernel, specifically in the io_uring subsystem, where a deadlock situation could occur due to incorrect sequence of acquiring locks. This vulnerability has been assigned the identifier CVE-2024-41080. In this blog post, we will explore the details about this vulnerability and its resolution, as well as discuss the importance of avoiding deadlocks in the kernel.

Background on the Issue

The io_uring subsystem is a powerful asynchronous I/O framework introduced in the Linux kernel 5.1 release. It offers a higher level of performance and scalability compared to the traditional AIO (Asynchronous I/O) interface. However, it is also susceptible to potential deadlocks if proper care isn't taken while acquiring and releasing locks.

The vulnerability in question was found in the io_register_iowq_max_workers() function where io_put_sq_data() is called, and the sqd->lock gets acquired without releasing the ctx->uring_lock. This can lead to a deadlock situation and create issues with the overall stability of the system. The issue was discovered in the process of investigating a similar problem in commit 009ad9fc6ee ("io_uring: drop ctx->uring_lock before acquiring sqd->lock").

Code Snippet of the Original Issue

static int io_register_iowq_max_workers(struct io_ring_ctx *ctx, void __user *arg)
{
    ...
    ctx->user_max_iowq_workers = uval;

    io_cqring_arm_signals(ctx);

    if (ctx->sq_data->max_iowqs)
        io_put_sq_data(ctx->sq_data, ctx->user_max_iowq_workers);
    ...
}

Resolution

To address the deadlock issue, developers have decided to release the ctx->uring_lock before calling io_put_sq_data() and then reacquire it after the function call. This ensures that the locks are acquired in the correct order, thus preventing the possibility of a deadlock in this part of the code.

Code Snippet Showing the Resolution

static int io_register_iowq_max_workers(struct io_ring_ctx *ctx, void __user *arg)
{
    ...
    ctx->user_max_iowq_workers = uval;

    io_cqring_arm_signals(ctx);
    mutex_unlock(&ctx->uring_lock);

    if (ctx->sq_data->max_iowqs)
        io_put_sq_data(ctx->sq_data, ctx->user_max_iowq_workers);

    mutex_lock(&ctx->uring_lock);
    ...
}

1. The original patch submitted by the developer to fix the issue.
2. The commit showing the resolution of the issue in the Git repository of Linux kernel.

Exploit Details

As of now, no known exploits have been reported targeting this specific deadlock vulnerability. However, deadlocks in the kernel could lead to system performance degradation or even crashes if triggered. Therefore, it is essential to take preventive measures by applying the necessary patches and keeping the kernel up to date.

Conclusion

CVE-2024-41080 highlights the importance of responsible handling of locks in the kernel to avoid deadlock situations. It also demonstrates the dedication and commitment of the Linux kernel development community to continuously seek out and address vulnerabilities that could affect the stability and security of the system. As users, it is our responsibility to keep ourselves informed and apply the necessary updates to ensure the smooth operation of our systems.

Timeline

Published on: 07/29/2024 15:15:15 UTC
Last modified on: 08/22/2024 13:39:43 UTC