CVE-2023-21264 - Exploiting a Memory Access Bug in mem_protect.c for Local Privilege Escalation

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Security researchers and system admins need to stay alert for CVE-2023-21264, a vulnerability in Android's memory protection code (mem_protect.c). This bug, caused by a misplaced memory access check, lets a local attacker with System-level privileges potentially reach and modify hypervisor memory—leading straight to privilege escalation. In this post, we’ll break down what went wrong, show some code snippets, walk through potential exploitation, and point you to original references for fixing or learning more.

What Is CVE-2023-21264?

This vulnerability affects multiple functions within the file mem_protect.c, which is a part of the Android Common Kernel. Normally, this code is supposed to keep sensitive areas, like hypervisor memory, protected from unauthorized access. However, due to a logic bug—the memory check happening *after* the memory is accessed—local attackers (with System execution privileges) can reach memory they shouldn’t.

Attack Vector: Local, via a crafted app or malicious code running with ‘system’ permissions

- Impact: Escalate to higher privileges, compromise hypervisor memory, potential for further code execution

References

- Android Security Bulletin, June 2023
- CVE entry at NVD

The Bug, In Simple Terms

Here's the core issue: the code that’s supposed to check *if* you’re allowed to access a specific region of memory is checking after the memory access, not before. This is often due to bad ordering of conditions.

Pseudocode (bad logic)

// mem_protect.c
void access_memory(void *ptr, size_t size) {
    // 1. Access memory
    int value = *((int *)ptr);
    // 2. THEN check if we should have done that
    if (!is_permitted(ptr, size)) {
        return DENIED;
    }
    // Continue regular logic...
}

Here, value = *((int *)ptr); actually *dereferences* a pointer before the check runs. So even if the pointer refers to hypervisor or protected memory, an attacker gets in before any security fence.

The correct code should look like this

void access_memory(void *ptr, size_t size) {
    // 1. CHECK FIRST
    if (!is_permitted(ptr, size)) {
        return DENIED;
    }
    // 2. Only now, access memory
    int value = *((int *)ptr);
    // Continue regular logic...
}

A Look At Possible Exploit

Let’s say an attacker has gained system-level code execution—typical with malicious apps that exploit other bugs to escalate permissions. That attacker can call the vulnerable function with a pointer into hypervisor memory. Since the memory check comes *after* access, the system dutifully reads from or writes to the protected memory first.

Exploit example (C code)

// Let's suppose dev/hypervisor_mem is mapped at xffff000
#define HYPERVISOR_PTR ((void*)xffff000)

void try_exploit() {
    size_t size = sizeof(int);
    // Call the vulnerable function, which shouldn't let us read this
    int result = access_memory(HYPERVISOR_PTR, size);
    printf("Read from hypervisor: %08x\n", result);
}

In a real-world exploit, the attacker could do more than read: they might overwrite function pointers, mess with memory mappings, or escalate further.

Why Does This Matter?

Hypervisor memory is where the host system keeps sensitive data separating virtual machines (VMs), controlling hardware access, and protecting the system's integrity. Allowing a local, system-level attacker to read/write that memory can defeat these protections, landing the attacker full kernel or virtual machine monitor privileges—a major breach.

This bug does not require user interaction to trigger. If an attacker lands on your device (say, via another vulnerability exploited by an app), they can chain CVE-2023-21264 to take over at a higher privilege level.

Are you at risk?

- If you run affected Android kernels and have not applied the security bulletin for June 2023, you may still be vulnerable.

How is it fixed?

- The memory access check in mem_protect.c was moved *before* the memory access. Double-check your device or kernel provider to ensure you’ve got the latest patches.

See the fix:
- Android kernel commit (example, may differ by platform)

Final Thoughts

CVE-2023-21264 is one of those “simple bug, big impact” vulnerabilities. By checking permissions after the action, the code left a short but serious window for attackers to steal or corrupt highly sensitive memory. Always keep security logic at the very front of your functions, and be wary of post-access validation. Review your system, patch where needed, and stay updated with the latest security bulletins.

Timeline

Published on: 08/14/2023 21:15:00 UTC
Last modified on: 08/24/2023 15:31:00 UTC