Linux Kernel, Memory Leak Vulnerability, CVE-2025-21792 (Critical)

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How the Mentioned CVE Works:

CVE-2025-21792 is a critical memory leak vulnerability in the Linux kernel’s AX.25 protocol implementation. The issue arises when the `SO_BINDTODEVICE` socket option is used to bind an AX.25 device to a socket. When this option is set, the kernel fails to increment the reference count of the bound device, leading to a refcount leak in the `ax25_release()` function. This leak occurs because the refcount decrement logic added in commit `9fd75b66b8f6` assumes the refcount was incremented during device binding. However, the refcount is only incremented in `ax25_bind()` and not when `SO_BINDTODEVICE` is used. This results in a refcount underflow, causing memory corruption and potential denial of service (DoS) or privilege escalation.
The vulnerability is triggered when a socket bound to an AX.25 device is released, leading to a warning in the kernel logs and memory leaks. The issue was reported by Syzkaller, and the fix involves ensuring refcounts are properly incremented and decremented in `ax25_setsockopt()` when handling SO_BINDTODEVICE.

DailyCVE Form:

Platform: Linux Kernel
Version: Up to 6.13.0-rc4
Vulnerability: Memory Leak
Severity: Critical
Date: 02/26/2025

(End of form)

What Undercode Say:

Exploitation:

1. Exploit Code:

Craft a malicious AX.25 socket program that repeatedly binds and releases devices using SO_BINDTODEVICE. This can exhaust kernel memory, leading to a DoS condition.

int sock = socket(AF_AX25, SOCK_SEQPACKET, 0);
setsockopt(sock, SOL_SOCKET, SO_BINDTODEVICE, "ax0", 4);
close(sock);

2. Triggering the Bug:

Use Syzkaller or similar fuzzing tools to trigger the refcount leak and observe kernel logs for warnings.

3. Exploit Impact:

  • Memory corruption leading to kernel panic.
  • Potential privilege escalation if combined with other vulnerabilities.

Protection:

1. Patch Application:

Apply the kernel patch that fixes the refcount handling in ax25_setsockopt().

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
git cherry-pick <commit-hash>

2. Kernel Hardening:

Enable kernel hardening features like `CONFIG_REFCOUNT_FULL` to detect refcount overflows and underflows.

echo "CONFIG_REFCOUNT_FULL=y" >> /path/to/kernel/config

3. Monitoring:

Monitor kernel logs for refcount warnings using `dmesg`.

dmesg | grep "refcount_t: decrement hit 0"

4. Disable AX.25:

If AX.25 is not required, disable it in the kernel configuration.

echo "blacklist ax25" >> /etc/modprobe.d/blacklist.conf

5. References:

6. Tools:

  • Use `perf` to analyze kernel memory usage. 
    perf stat -e kmem:kmalloc

7. Mitigation Script:

Automate refcount monitoring with a custom script.

!/bin/bash
while true; do
dmesg | grep "refcount_t" && echo "Refcount issue detected!" >> /var/log/refcount.log
sleep 10
done

By following these steps, you can exploit, analyze, and protect against CVE-2025-21792 effectively.

References:

Reported By: https://nvd.nist.gov/vuln/detail/CVE-2025-21792
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