Linux is known for its robustness, but like any complex system, it occasionally trips over subtle bugs. In early 2024, the Linux kernel development community fixed a pair of memory leaks in the media subsystem, specifically within the Technotrend PCI (ttpci) driver code. This vulnerability, CVE-2024-27073, might sound obscure but highlights how even small mistakes can add up in low-level system software. In this deep dive, I’ll walk you through what happened, show you the critical code, and explain why resources should always be released on failure.

CVE-2024-27073 is a memory leak vulnerability patched in the Linux kernel. It was tracked as

> media: ttpci: fix two memleaks in budget_av_attach

The bug is about resource management in budget_av_attach, a critical function of the DVB subsystem, which handles digital TV tuner cards that use the Technotrend PCI chips. When device registration or video initialization fails (saa7146_register_device or saa7146_vv_init), the code should free all memory properly, but before the fix, it didn’t. Over time, this could lead to hard-to-track memory leaks, especially if kernel modules are loaded and unloaded repeatedly.

Let’s look at the problematic (pre-fix) code

static int budget_av_attach(struct saa7146_dev *dev, struct saa7146_pci_extension_data *pci_ext)
{
    ...
    result = saa7146_register_device(c, &budget_av_template, "budget_av");
    if (result < ) {
        // FIXME: deallocation is missing here!
        return result;
    }

    result = saa7146_vv_init(dev, &budget_av_vv_ops);
    if (result < ) {
        // FIXME: deallocation is missing here too!
        return result;
    }

    ...
}

The // FIXME comments literally existed to point out unhandled cleanup. If either saa7146_register_device() or saa7146_vv_init() failed, the function just returned, leaving allocated resources in memory. This leakage can accumulate every time the device initialization fails.

How Was It Fixed?

The fix did something that should be very common in kernel driver code: cleanup on error.

Developers updated error paths to call proper memory-freeing functions, mimicking the style of error handling used in the closely related ttpci_budget_init() code path.

Here’s what the corrected code looks like (simplified for clarity)

static int budget_av_attach(struct saa7146_dev *dev, struct saa7146_pci_extension_data *pci_ext)
{
    ...
    result = saa7146_register_device(c, &budget_av_template, "budget_av");
    if (result < ) {
        dvb_frontend_detach(...);
        kfree(c);
        return result;
    }

    result = saa7146_vv_init(dev, &budget_av_vv_ops);
    if (result < ) {
        saa7146_unregister_device(dev, ...);
        dvb_frontend_detach(...);
        kfree(c);
        return result;
    }
    ...
}

Notice how each failure case is now paired with calls to dvb_frontend_detach() and kfree(), deallocating whatever had been set up.

Why Does This Matter?

- Memory leaks in the kernel are dangerous: They eat up kernel memory (which is precious), and over time can degrade system performance or cause out-of-memory (OOM) errors.
- Attackers and stability: While this particular bug doesn’t directly allow privilege escalation, persistent memory leaks may be used as part of a denial-of-service (DoS) scenario, especially in systems allowing unprivileged kernel module manipulation.

There’s no public exploit for this yet, but here’s a straightforward idea

1. Repeatedly attempt to initialize a device using the buggy function, ensuring error paths are taken (simulating device init failures).
2. Watch /proc/meminfo or slabtop as you do it. Each time, kernel memory is leaked.
3. Eventually, the system may experience OOM problems, or instability, especially on embedded systems where resources are tight.

Here’s a small C code sketch that simulates the repeated loading/unloading (though real exploit would require driver-level access):

# Not actual exploit, just demonstration pseudocode

modprobe -r budget_av
for i in {1..100}; do
    modprobe budget_av fake_init_error=1
    modprobe -r budget_av
done

This would trigger the leaks (if run on the buggy kernel and matching hardware), resulting in steadily increasing kernel memory consumption.

- Commit fixing CVE-2024-27073 (lkml.org)
- Linux Kernel source – budget-av.c
- CVE-2024-27073 at MITRE
- Upstream patch discussion

The Linux community polices its own code, and even old "FIXME" comments eventually get resolved.

- If you use digital TV cards with Linux kernels before the fix, upgrade to avoid long-term resource leaks.


### Stay tuned for more breakdowns of kernel vulnerabilities – and remember to always check the error path!

Timeline

Published on: 05/01/2024 13:15:51 UTC
Last modified on: 11/06/2024 19:35:15 UTC