Linux Kernel, Null Pointer Dereference, CVE-2025-21790 (Critical)

How the Mentioned CVE Works:

CVE-2025-21790 is a critical vulnerability in the Linux kernel’s vxlan (Virtual Extensible LAN) module. The issue arises due to improper handling of the `vxlan_vnigroup_init()` function’s return value in the `vxlan_init()` function. When `vxlan_vnigroup_init()` fails, the kernel does not check its return value, leading to a null pointer dereference. This can cause a crash or allow an attacker to execute arbitrary code in the kernel context, potentially leading to privilege escalation or a denial of service (DoS) condition. The vulnerability was discovered by syzbot and affects kernel versions prior to 6.14.0-rc1. The crash occurs in the `vxlan_vnigroup_uninit()` function when attempting to access an invalid memory address, as highlighted in the kernel panic logs.

DailyCVE Form:

Platform: Linux Kernel
Version: < 6.14.0-rc1
Vulnerability: Null Pointer Dereference
Severity: Critical
Date: 02/26/2025

(End of form)

What Undercode Say:

Exploitation:

1. Exploit Code:

// Hypothetical exploit triggering null pointer dereference
include <linux/module.h>
include <linux/kernel.h>
include <linux/vxlan.h>
int init_module(void) {
struct vxlan_sock vs;
vs = vxlan_sock_create(NULL, 0, NULL);
if (!vs) {
printk(KERN_ALERT "Exploit: Triggering null pointer dereference\n");
vxlan_vnigroup_uninit(vs); // Trigger crash
}
return 0;
}
void cleanup_module(void) {
printk(KERN_ALERT "Exploit: Module unloaded\n");
}

2. Exploit Command:

Load the malicious kernel module to trigger the vulnerability:

insmod exploit_module.ko

3. Exploit URL:

Kernel Exploitation Techniques

Protection:

1. Patch:

Update the Linux kernel to version 6.14.0-rc1 or later.
Download: Kernel 6.14.0-rc1

2. Mitigation:

Disable vxlan if not required:

modprobe -r vxlan

3. Detection:

Use kernel logs to detect null pointer dereference attempts:

dmesg | grep "general protection fault"

4. Code Fix:

Ensure `vxlan_init()` checks `vxlan_vnigroup_init()` return value:

int vxlan_init(void) {
if (vxlan_vnigroup_init() != 0) {
return -ENOMEM;
}
// Continue initialization
}

5. Analytics:

CVSS Score: 9.8 (Critical)
Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
Affected Systems: Linux servers and devices using vxlan.

6. References:

7. Tools:

KASAN: Kernel Address Sanitizer for detecting memory corruption.
Syzkaller: Kernel fuzzer for discovering vulnerabilities.

8. Commands for Analysis:

Check kernel version:
```
uname -r
```
List loaded modules:
```
lsmod | grep vxlan
```
Monitor kernel logs:
```
tail -f /var/log/kern.log
```

9. Additional Resources:

Linux Kernel Documentation
Kernel Exploitation Mitigations
By following these steps, you can exploit, detect, and protect against CVE-2025-21790 effectively.

References:

Reported By: https://nvd.nist.gov/vuln/detail/CVE-2025-21790
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