CVE-2022-20109 - Deep Dive into the Ion Use-After-Free Vulnerability (Android Escalation of Privilege)

Android's security is critical for billions of users worldwide, but every year new vulnerabilities are found in its system components. One such bug, CVE-2022-20109, came up in the Ion memory allocation subsystem. It's notable because it allows a local attacker (basically any app running on your phone) to get more privileges than intended — and without needing any user interaction. Let’s break down what’s going on, how exploitation works, and what was done to fix it.

What Is Ion?

Ion is a memory allocator used widely in many Android devices for allocating shared memory. It allows the kernel and user space processes (like apps) to share memory buffers, often for things like graphics and multimedia.

The Vulnerability: Use-After-Free from Reference Count Mismanagement

In the reported bug, Ion didn’t properly update reference counts when freeing memory. Simply put, a reference count tracks how many places are using a chunk of memory. When it hits zero, the system knows it’s safe to free that memory.

But in this case, Ion could accidentally free memory while something else was still using it — a classic use-after-free issue. This can be exploited by a local attacker to hijack memory for their own purposes, usually to escalate their privileges on the system.

References

* CVE Reference at NIST
* Android Security Bulletin
* AOSP Commit Log (search for ALPS06399915)

Real-World Impact

- Escalation of privilege: Malicious apps can use this bug to break out of their sandbox and get root or system-level privileges on the phone.

Code Walkthrough: Where the Bug Lived

The following is a simplified example similar to the buggy code pattern. Here’s how a use-after-free could happen:

// Simplified pseudo-code illustrating the bug

struct ion_buffer {
    atomic_t ref_count;
    void *data;
};

void ion_free(struct ion_buffer *buffer) {
    // DANGER: Only checks ref_count here
    if (atomic_dec_and_test(&buffer->ref_count)) {
        kfree(buffer->data);   // Free data when ref count hits zero
        kfree(buffer);         // Free the buffer itself
    } else {
        // Someone else is still using the buffer.
        // But race condition here if not protected properly.
    }
}

void another_function(struct ion_buffer *buffer) {
    // Exploiter triggers release in another thread
    ion_free(buffer);  // Buffer might be freed here

    // ... meanwhile, on this thread ...
    use_buffer(buffer); // Use-after-free if buffer is already freed
}

Problem:
If two processes decrement the reference count at just the right time, both might see a non-zero count before one finally hits zero, allowing both to use the (now freed) memory, leading to undefined behavior and a potential path to privilege escalation.

Exploit Details

This is a local privilege escalation bug. Here’s how a simplified exploit scenario works (as seen in some public PoCs on past Ion bugs):

Malicious app allocates an Ion buffer and makes multiple references to the same buffer.

2. App manipulates the reference count (through race conditions, often using threads or direct ioctl calls) so that Ion incorrectly frees the buffer while still in use *elsewhere*.
3. After the buffer is freed, attacker quickly reallocates that now-unprotected memory (often by spraying the heap).
4. Since the freed memory is being used as a "buffer" elsewhere, app now controls what data is processed at a high privilege context.

This can allow privilege escalation, for example by crafting data in the buffer that the kernel interprets as important metadata or pointers.

Example PoC pattern

# Pseudo-code for illustrative purposes (actual PoCs are more complex and device-specific)
import threading

buffer_handle = ion_alloc(...)  # allocate buffer

# Start two threads to race buffer release
def release():
    for _ in range(100):
        ion_free(buffer_handle)

t1 = threading.Thread(target=release)
t2 = threading.Thread(target=release)

t1.start()
t2.start()
t1.join()
t2.join()

# Try to allocate fake buffer at same address
spray_data = custom_payload()
for _ in range(10000):
    allocate_buffer_with_payload(spray_data)

*Note: This is conceptual. Real attacks might use custom kernel modules and more precise timing.*

Patch & Mitigation

The patch for ALPS06399915 (referenced in AOSP) improved the handling of reference counting to make sure memory can never be freed while still in use elsewhere.

Patch Snippet (simplified)

// In the real fix, access to buffer objects is properly protected with locking
mutex_lock(&buffer->lock);

if (atomic_dec_and_test(&buffer->ref_count)) {
    kfree(buffer->data);
    kfree(buffer);
}

mutex_unlock(&buffer->lock);

This guarantees that no race condition exists, and frees memory only when it’s truly not in use. Always keep your devices updated to get this fix.

Conclusion

CVE-2022-20109 is a great example of how a tiny error in memory management (like reference counting) can have big consequences for device security. Any app could potentially abuse this type of bug to gain complete control over an Android device.

Timeline

Published on: 05/03/2022 20:15:00 UTC
Last modified on: 05/11/2022 15:50:00 UTC