CVE-2025-68233: The Zombie PID Apocalypse in Tegra Drivers

If you work with embedded AI, robotics, or automotive systems, you know the NVIDIA Tegra series. It's the brain inside a lot of cool hardware. To make that hardware sing, the Linux kernel uses the Tegra DRM (Direct Rendering Manager) driver. It handles the graphics pipeline, talking to the hardware synchronization engine known as host1x.

But here's the thing about complex drivers: they are often a messy web of memory allocations, context switching, and reference counting. And where there is manual reference counting, there is human error.

CVE-2025-68233 isn't a flashy remote code execution bug that will let hackers steal your crypto wallet. It's something more insidious. It's a resource leak. Specifically, a PID reference leak. Think of it like opening a tab at a bar, ordering a drink, and then the bartender forgets to ever close the tab. You leave, but the tab stays open forever. Do that enough times, and the bar runs out of paper.

To understand this bug, you have to understand how the Linux kernel tracks processes. When a driver needs to associate an operation with a specific process, it often deals with struct pid. To ensure this structure doesn't vanish while the driver is looking at it, the kernel uses reference counting.

The function get_task_pid(task, type) does two things: it finds the PID structure for a task and increments its usage counter. This is the kernel saying, "I am using this object, don't delete it yet."

The rule of law in kernel development is simple: If you get, you must put.

The Tegra driver developers broke this law. In the path for creating a memory context (tegra_drm_ioctl_context_create), they called get_task_pid() to pass the PID to host1x_memory_context_alloc(). However, host1x_memory_context_alloc() doesn't take ownership of that reference; it just uses it. Once that function returned, the driver continued on its merry way, completely forgetting to call put_pid(). The result? A struct pid that the kernel thinks is still in use, forever.

Let's look at the "smoking gun" in drivers/gpu/drm/tegra/drm.c. It's a classic one-line fix, which usually implies a face-palm moment for the developer.

Here is the vulnerable logic before the patch:

// The vulnerable code path
pid = get_task_pid(current, PIDTYPE_PID);
 
// The PID is passed here, but ownership is NOT transferred
context = host1x_memory_context_alloc(tegra->host1x, pid);
 
// ... The code just continues, and 'pid' is leaked.
if (IS_ERR(context))
    return PTR_ERR(context);

The fix is painfully simple. They just needed to release the reference immediately after using it. Here is the patch applied in commit 6cbab9f0da...:

  pid = get_task_pid(current, PIDTYPE_PID);
  context = host1x_memory_context_alloc(tegra->host1x, pid);
+ put_pid(pid); // <--- The Savior Line
  if (IS_ERR(context))
      return PTR_ERR(context);

Without that put_pid(pid), every time you create a context, a small chunk of kernel memory is allocated and never freed. It’s a slow bleed, but bleed long enough and you die.

How do we weaponize this? We don't need a buffer overflow or a ROP chain. We just need a while(1) loop.

The attack vector is the DRM_IOCTL_TEGRA_CONTEXT_CREATE IOCTL. This is accessible to anyone with access to the render node (usually /dev/dri/renderD128 or similar), which typically includes any user in the video or render group.

The Attack Scenario:

1. Open the Driver: The attacker opens a file descriptor to the Tegra DRM device.
2. Spam the Kernel: The attacker enters a loop, repeatedly calling the context creation IOCTL.
3. Watch the World Burn: Each iteration leaks a struct pid. While struct pid is small, the kernel slab allocator will keep allocating new pages to hold them. Eventually, you exhaust kernel memory (specifically the slab cache).

// Conceptual PoC
int fd = open("/dev/dri/card0", O_RDWR);
struct drm_tegra_open_channel_args args = {0};
 
while(1) {
    // This call triggers get_task_pid(), leaks it, and returns.
    // We don't even care if it succeeds or fails, as long as it hits the leak.
    ioctl(fd, DRM_IOCTL_TEGRA_CONTEXT_CREATE, &args);
}

This is a classic Denial of Service. On an embedded device like a Jetson Nano running a robot, this effectively freezes the system or triggers the OOM killer, likely taking down critical processes.

Since this is a kernel-level bug in a specific driver, you can't just change a config file or add a firewall rule. The code is broken.

Remediation Strategy:

1. Update the Kernel: This is the only real fix. The Linux stable teams have backported this to 6.1, 6.6, 6.12, and others. If you are building Yocto images for Tegra devices, ensure you pull the latest stable branch.
2. Restrict Access: If you can't patch immediately, restrict access to /dev/dri/card* and /dev/dri/render*. If untrusted users can't touch the DRM device, they can't trigger the leak. However, this might break your graphics stack (Wayland/X11).

The patch is already merged in mainline Linux (v6.18) and backported to stable trees. If you are running a custom vendor kernel (common in the embedded world), you might need to cherry-pick commit 6cbab9f0da72b4dc3c3f9161197aa3b9daa1fa3a manually.