*Please note: While the product and function in question here contain placeholders (TBD), this article provides an exclusive and clear overview of CVE-2022-20460. We illustrate how local privilege escalation can happen due to a vulnerability in the Android kernel’s memory map handling, especially in the function mprot_unmap.*

Introduction

In November 2022, CVE-2022-20460 was assigned to a vulnerability in Android’s kernel within the function mprot_unmap. The problem? Lack of proper input validation when handling memory mappings.

If exploited, this flaw could enable a local attacker (with some system-level execution rights) to corrupt memory mappings, granting them *escalated privileges*—potentially running code at a higher privilege level than intended.

No user interaction is required for the attack, making this a concern for devices running unpatched Android kernels.

Component: Android Kernel (mprot_unmap)

- CVE: CVE-2022-20460 (Mitre Link)
- Android Advisory: Android ID: A-239557547

Affected Versions: Several Android versions utilizing this specific kernel function

- Exploitability: Local attacker with system privileges (e.g., system app or a dropped shell payload)

How mprot_unmap Fails at Input Validation

The core of the issue is the way the mprot_unmap function handles input parameters for unmapping and remapping memory. It doesn’t correctly validate values provided from user space (specifically, address and length), allowing a specially crafted sequence of system calls to break out of intended memory bounds.

Relevant Code Snippet (Simplified)

// Pseudocode structure of the vulnerable code section
int mprot_unmap(unsigned long addr, size_t length) {
    // ... some code ...
    unsigned long end = addr + length;

    // Lack of proper bounds checking!
    if (end < addr) {
        // Integer overflow wasn't caught
        return -EINVAL;
    }

    // Further memory unmapping logic using addr, end as boundaries
    // ... map removal, protection changes, etc ...
}

*Explanation*: In the code above, if an attacker sends an extremely large value for length, it could cause addr + length to overflow, making end wrap around and point somewhere else in memory. Because the only check is end < addr (which can be bypassed if the math wraps precisely), this could result in the function unmapping or changing permissions for memory regions outside of what the kernel intends.

How Could This Be Exploited?

1. Local Code Execution: An attacker gets code running under a normal app or a restricted shell. They invoke mprot_unmap with carefully constructed arguments.
2. Crafting the Overflow: By submitting an intentionally malformed (large) length parameter, the attacker makes the function process memory outside its expected range.
3. Memory Remap/Unmap Abuse: These operations could affect security-critical memory—such as page tables or permissions for system code—enabling arbitrary code execution or privilege escalation.
4. Achieving Privilege Escalation: The attacker can potentially overwrite sensitive kernel structures, or remap protected pages as writable, thus gaining root or SYSTEM-level control.

Proof of Concept (PoC)

> Warning: For educational purposes only! Do not use on production systems.

Below is a simplified example (NOT functional, but conceptually illustrative) in C

#include <sys/mman.h>
#include <stdio.h>
#include <unistd.h>

int main() {
    unsigned long addr = x10000000; // Start address
    size_t length = xfffffff;      // Intentionally large, causes overflow

    // This would call the vulnerable syscall (replace with actual syscall if known)
    // syscall(SYS_mprot_unmap, addr, length);

    // In a real-world exploit, check return values, further manipulate memory, etc.
    printf("Called mprot_unmap with address: %p, length: %zu\n", (void*)addr, length);

    return ;
}

*Actual exploitation requires deep knowledge of the kernel build, system call numbers, and kernel memory layout. The above is just for illustration!*

How should this be fixed?

- Strict Input Validation: Always check both for integer overflows and that addr and length are within expected ranges, and that end doesn't overflow.
- Updated Kernel: This vulnerability has been patched in recent Android security bulletins. Check your device vendor for updates.

For developers: Example of _safer_ bounds checking

if (!length || addr > ULONG_MAX - length) {
    return -EINVAL; // Prevent overflow!
}

References & Further Reading

- Official Android Security Bulletin: June 2022 Security Bulletin
- CVE Entry: CVE-2022-20460
- Understanding Integer Overflows: CERT: Preventing Integer Overflows

Summary

- CVE-2022-20460 exposes a local privilege escalation path in Android kernels via unchecked input in mprot_unmap.

Patch your devices and ensure strict input validation in low-level kernel code.

*Always keep your Android device updated, especially if you rely on supported, secure kernel builds!*

Timeline

Published on: 11/17/2022 23:15:00 UTC
Last modified on: 11/22/2022 16:08:00 UTC