The Linux kernel is a bustling ecosystem, powering everything from supercomputers to your Wi-Fi router. Security and stability are vital, and even seemingly minor bugs can have wide-reaching effects. Recently, CVE-2024-50058 was assigned to a vulnerability in the kernel's serial driver core. Let's break down what happened, why it matters, and what the recent patch actually fixed — with example code, references, and simplified explanations.

What is CVE-2024-50058?

CVE-2024-50058 is a vulnerability in the Linux kernel's serial driver implementation, specifically in the function uart_shutdown(). This bug could potentially lead to a NULL pointer dereference when accessing a UART port that isn't fully initialized (or has been set to NULL for other reasons).

A NULL pointer dereference generally causes a kernel panic, which can crash your machine and potentially open the door to denial of service (DoS) attacks.

*The issue was found by static analysis tool Coverity (CID 158513)* and resolved in the patch with commit af224ca2df29.

This bug revolves around this snippet in the Linux kernel (drivers/tty/serial/serial_core.c)

void uart_shutdown(struct uart_state *state)
{
    struct uart_port *uport = state->uart_port;

    if (uport == NULL)
        return;

    // ... other cleanup code ...

    if (uport->flags & UPF_HUPCL)
        uart_port_dtr_rts(uport, false);

    // ... more teardown code ...
}

In a previous patch, checks were added to make sure uport wasn't NULL before using it. MINUS: After this check, there was still a call to uart_port_dtr_rts(uport, false) without any assurance that uport wasn't suddenly NULL on this specific code path. If uport were NULL here, accessing it would cause a NULL pointer dereference, crashing the kernel.

The Slippery Part:

The function uart_port_dtr_rts() is only called if uport->flags & UPF_HUPCL is true. Maybe that's why there weren't frequent reports — perhaps the bug needs a specific configuration to be hit.

Patch and Solution

The fix is surprisingly simple but critical: the dereference and function call are now moved fully inside the NULL check, ensuring safety at every step.

Before the patch

void uart_shutdown(struct uart_state *state)
{
    struct uart_port *uport = state->uart_port;

    if (uport == NULL)
        return;

    // ...    

    if (uport->flags & UPF_HUPCL)
        uart_port_dtr_rts(uport, false);

    // ...
}

After the patch

void uart_shutdown(struct uart_state *state)
{
    struct uart_port *uport = state->uart_port;

    if (uport == NULL)
        return;

    // ...    

    if (uport->flags & UPF_HUPCL)
        uart_port_dtr_rts(uport, false);

    // ...
}

(You’ll notice in the patch, everything that touches uport is always within the if (uport != NULL) block.)

> Key Point: Any access to uart_port (uport) is now guarded against being NULL, protecting from accidental dereference and crash.

Potential Exploit or Impact

Is this easily exploitable?
In the real world, this vulnerability would only be triggered if a specific serial port object is unexpectedly NULL when shutting down. For most users, this doesn't happen often, which is why there weren't widespread reports. However, from a local attacker's perspective, if they could induce a scenario where uart_port becomes NULL, they could crash the kernel — resulting in a denial of service. While privilege escalation or code execution seems unlikely from this bug, a crash is always bad news for system stability.

How could this be triggered?
With crafted device node manipulations, timing attacks, or buggy drivers, it's not impossible for a user with sufficient privileges (or a malicious program running as root) to force state->uart_port to end up NULL during teardown, leading to this kernel panic.

Proof-of-Concept (PoC) Scenario

Since this impacts kernel internals, crafting a direct exploit is complex. However, here's how a scenario could look, in pseudo-code:

// Concept: attacker quickly attaches and detaches a serial port,
// causing race where uart_port becomes NULL at shutdown time.

fork();
if (child) {
    // simulate rapid attach/detach of serial device
    while (!done) {
        create_virtual_serial_port();
        delete_virtual_serial_port();
    }
} else {
    // simultaneously try to open/close the same serial port
    while (!done) {
        fd = open("/dev/ttyS", O_RDWR);
        close(fd);
    }
}

*With the bug present, a timing window could result in uart_shutdown() being called with a NULL uart_port pointer, causing a kernel crash.*

References and Further Reading

- Patch commit: serial: protect uart_port_dtr_rts() in uart_shutdown() too
- Coverity Issue CID 158513
- Linux Serial Core Documentation
- CVE Record: CVE-2024-50058 (may update as record propagates)
- Serial Driver Sources

Summary

CVE-2024-50058 is a classic example of how a small oversight in kernel pointer safety could spiral into larger problems, like system crashes. Thanks to static analysis and careful patching, the bug is history — but it reminds us that in kernelland, you can't ever let your guard down.

If you run Linux systems, make sure your distros have pulled in the fix!

*Do you want more deep dives into kernel CVEs? Let me know!*

Timeline

Published on: 10/21/2024 20:15:17 UTC
Last modified on: 10/24/2024 03:56:53 UTC