CVE-2022-4139 - Linux Kernel i915 Driver TLB Flush Flaw Explained (with Exploit Details)

In late 2022, security researchers discovered a critical bug in the Linux kernel’s Intel GPU driver (i915), tracked as CVE-2022-4139. This flaw, caused by an incorrect TLB (Translation Lookaside Buffer) flush, can allow a local attacker to crash the system, access privileged memory, or even escalate their privileges. That means someone with basic access to a Linux machine could possibly take it over. Let's break down what this means, how it works, and see some code showing the problem—and how attackers might exploit it.

What is the i915 Driver?

The i915 driver is the core Intel Graphics driver for modern Linux systems, powering millions of PCs, laptops, and workstations. It handles everything from drawing your desktop to processing CUDA or Vulkan GPU tasks.

How Does the Bug Happen?

Every process on your Linux box has its own view of memory, enforced by something called the MMU (Memory Management Unit). The MMU uses page tables, and every time a process gets or releases access to GPU memory, these tables need updating. The TLB is a CPU cache of these translations. If you don’t flush it correctly, an old, now-invalid entry can hang around.

CVE-2022-4139 happened because the i915 GPU driver neglected to flush the TLB at the right time when "GEM" object mappings changed. That means a process could still access memory it shouldn’t, or get data from someone else’s process, even kernel memory in some cases. That’s a huge problem.

A Peek at the Problematic Code

Here’s a simplified version illustrating where things went wrong. In the i915 driver, when unmapping a memory area, there should be an explicit flush to remove stale page-table entries. In affected versions (before the fix), it looked like this:

// Vulnerable snippet (do NOT use in production!)
void i915_gem_object_unmap(struct drm_i915_gem_object *obj) {
    // ...unmap object from user space
    unmap_pages(obj);  // removes page mappings
    // <--- MISSING: TLB flush here!
}

Instead, it should be

// Fixed code
void i915_gem_object_unmap(struct drm_i915_gem_object *obj) {
    unmap_pages(obj);
    flush_tlb(obj);    // flushes any remaining stale address mappings from the TLB!
}

Because the TLB wasn’t cleared, an attacker could keep using a pointer to access freed memory, which might now belong to another process—or even the kernel.

Exploit Details

Threat Model:

Map a GEM object (GPU memory buffer) into user space.

2. Unmap/release the object, but keep pointers/handles to it.

Proof-of-Concept Script (Illustrative, Not Weaponized)

// This pseudo-code shows the attack flow.
// Real exploitation is harder, but this is the logic:

int main() {
    int handle = drm_gem_create();     // Allocate a buffer
    void *addr = drm_gem_map(handle);  // Map it

    drm_gem_unmap(handle);             // Unmap it in kernel

    // ...Fast, force a new object to take its place
    int handle2 = drm_gem_create();  

    // Exploit: addr still points to valid memory! Try reading/writing:
    printf("Leaked data: %s\n", (char*)addr);

    // Could trigger kernel panic or privilege escalation here
}

Impact

- Privilege Escalation: In real-world scenarios, attackers can gain root by corrupting kernel memory structures.

Fix and Mitigation

- The official patch can be read here.

References

- CVE-2022-4139 Summary - NVD
- Upstream Linux Patch
- Red Hat Security Advisory
- Ubuntu Security Notice

Final Thoughts

CVE-2022-4139 is a great example of how subtle bugs in GPU drivers can have massive security impacts. If you run a Linux machine with Intel graphics, updating your kernel is critical. Linux’s complexity, especially around GPUs, makes TLB flush bugs like this easy to miss—but devastating if exploited.

Stay secure: Always keep your systems patched, and don’t ignore kernel updates!

*(This post is exclusive content for educational purposes—do not use for unauthorized access. Always follow your organization’s security policies!)*

Timeline

Published on: 01/27/2023 18:15:00 UTC
Last modified on: 03/09/2023 19:15:00 UTC