CVE-2021-0883 is a security flaw discovered in the PowerVR kernel driver, a critical component running deep within many Android devices’ hardware. This bug is tied to the “PVRSRVBridgeCacheOpQueue” function. A missing check on a size value results in a nasty integer overflow, which an attacker can exploit to access out-of-bounds memory on the heap. This issue could eventually allow a local attacker to escalate privileges — all without requiring any user action.

Let’s break down what’s really going on, how this bug works under the hood, and what makes it dangerous on Android devices leveraging PowerVR GPU technology.

What is the PowerVR Driver and Why is it Important?

PowerVR is an ultra-common GPU architecture found in plenty of mobile devices worldwide, including Android smartphones. Its “kernel driver,” found in the Android kernel space, bridges between the operating system and the graphics hardware for tasks such as rendering, display control, and cache management.

Kernel drivers have high-level privileges by nature. If something goes wrong at this level, malware or attackers can use it to elevate their own access — potentially taking control of the entire system.

The Vulnerable Function: PVRSRVBridgeCacheOpQueue

The bug lives in the function PVRSRVBridgeCacheOpQueue. This function is responsible for managing cache operations, and processes requests from user space code.

The vulnerability arises from missing validation on a user-controlled size value. Specifically, there’s nothing stopping an attacker from passing a huge value, causing a calculation to wrap around and become small (due to integer overflow), resulting in the allocation of a buffer that’s way too small. When the code later tries to write data beyond this buffer, you get classic out-of-bounds heap access.

This is a textbook integer overflow (CWE-190), and it’s especially dangerous given where it’s happening.

Simple Breakdown: How the Bug Works

1. Attacker opens a handle to /dev/pvrsrvkm (the kernel driver’s device node).
2. Attacker crafts a malicious IOCTL request to trigger PVRSRVBridgeCacheOpQueue, setting a large integer in the size value.
3. The kernel driver code calculates a buffer size using this value, but doesn’t check for overflow.

When the integer wraps (e.g., xFFFFFFFF + 1 = ), the allocation is much too small.

5. Next, the code copies user data into the tiny buffer, writing way past the bounds of the heap buffer.
6. Heap corruption occurs, which, with some effort, can be turned into code execution (privilege escalation) in the kernel context.

Here’s a simplified example of the vulnerable pattern in C

// Simplified vulnerable function
int PVRSRVBridgeCacheOpQueue(unsigned int size, const char *user_buf) {
    // Suppose size is attacker-controlled, no bounds check!
    char *buf = malloc(size * sizeof(CacheOp));
    if (!buf) return -ENOMEM;

    // Dangerous - user_buf gets copied into potentially too-tiny buffer!
    if (copy_from_user(buf, user_buf, size * sizeof(CacheOp))) {
        free(buf);
        return -EFAULT;
    }

    // ... rest of function ...

    free(buf);
    return ;
}

If size is huge (like x40000000), size * sizeof(CacheOp) might overflow, causing a small buffer to be allocated — but you copy way too much. This triggers the heap overflow.

How Can an Attacker Use This?

- Gaining Root on Android: Because the bug is in a kernel driver, it’s in the attack path for kernel exploitation. Attackers may use this for rooting the device—escalating normal app privilege to full Administrator.

Persistent Malware: Malicious payloads could be installed with full device privileges.

- Bypassing Sandboxes: Android’s app isolation can be broken, accessing data of other apps or even the OS.

How Is This Exploited? (Flow Example)

1. Open /dev/pvrsrvkm device node

Trigger Overflow: Data written beyond heap-bound buffer in kernel space

5. Hijack Execution: Use heap corruption to overwrite function pointers, jump to ROP chain, spawn root shell, etc.

Original References

- NVD Record: https://nvd.nist.gov/vuln/detail/CVE-2021-0883
- Google Android Advisory: https://source.android.com/security/bulletin/2021-11-01
- AOSP Bug Report (Android ID: A-270395013): https://android.googlesource.com/platform/hardware/imagination/+/b99147cc21794639cba5b687fc1d48d7d36be1e

How Was It Fixed?

Google and partners patched the issue by adding the missing size validation, so overflowing buffer sizes can’t sneak through anymore. The fix checks that the size is within safe bounds before any allocation or copying is performed.

Who’s at Risk?

Any Android device relying on a vulnerable version of the PowerVR kernel driver is at risk. The attack is local (must be run on the device) and can be triggered by apps that don’t have special permissions.

Recommendations

- Update Your Phone: Ensure your Android is running at least the November 2021 security update (or newer).
- Be Careful With Unknown Apps: Don't sideload APKs from untrusted sources, as local privilege escalation bugs often pair with other malware.

Conclusion

CVE-2021-0883 is a classic “small bug, big consequences” story—a missing size check in the PowerVR kernel driver could allow anyone with local access to break Android’s security model wide open. It’s another reminder that critical code paths (especially in the kernel) must treat all input as untrusted—and validate, validate, validate.


If you’re a developer maintaining kernel code, always check user-controlled values before allocating or copying memory. This simple best practice would have stopped this bug before it started.


For further reading, see the NVD entry for CVE-2021-0883, and Android Security Bulletins.

Timeline

Published on: 04/19/2023 20:15:00 UTC
Last modified on: 04/27/2023 20:32:00 UTC