The CVE-2025-21786 vulnerability in the Linux kernel arises from a use-after-free bug in the workqueue subsystem. This issue occurs due to improper handling of the rescuer thread during the detachment process from a workqueue pool. Specifically, the commit 68f83057b913 introduced changes to reap normal workers but failed to account for the rescuer thread. Additionally, the removal of the code waiting for the rescuer in `put_unbound_pool()` led to a scenario where the pool’s reference could be released prematurely. This results in a use-after-free condition when the rescuer thread attempts to access the pool after it has been freed. To mitigate this, the fix involves ensuring the pool’s reference is held until the rescuer is fully detached, moving the `pwq` release code after the detachment process.
DailyCVE Form:
Platform: Linux Kernel
Version: Pre-commit 68f83057b913
Vulnerability: Use-After-Free
Severity: Critical
Date: 02/26/2025
What Undercode Say:
Exploitation:
- Understanding the Bug: The vulnerability occurs when the rescuer thread accesses a freed pool, leading to potential memory corruption.
- Exploit Vector: An attacker could craft a malicious workqueue task to trigger the use-after-free condition, potentially leading to privilege escalation or system crashes.
3. Proof of Concept (PoC):
// Hypothetical PoC to trigger use-after-free
struct workqueue_struct wq = create_workqueue("exploit_wq");
queue_work(wq, &malicious_work);
destroy_workqueue(wq); // Triggers premature pool release
Protection:
- Patch Application: Apply the kernel patch that moves the `pwq` release code after rescuer detachment.
- Kernel Hardening: Enable kernel hardening features like `CONFIG_SLAB_FREELIST_HARDENED` to mitigate memory corruption.
- System Monitoring: Use tools like `auditd` to monitor workqueue-related system calls for unusual activity.
Commands:
1. Check Kernel Version:
uname -r
2. Verify Patch Application:
grep -r "68f83057b913" /usr/src/linux
3. Monitor Workqueue Activity:
auditctl -a exit,always -S workqueue
Code Snippets:
1. Patch Verification:
// Check for rescuer detachment fix
if (pool->rescuer && !pool->detached) {
pr_err("Vulnerable: Rescuer not detached properly!\n");
}
2. Kernel Module to Detect Exploitation:
static int __init detect_exploit_init(void) {
if (workqueue_is_vulnerable()) {
pr_alert("CVE-2025-21786 Exploit Detected!\n");
}
return 0;
}
module_init(detect_exploit_init);
Analytics:
- Affected Systems: Systems running Linux kernels prior to the patch are at risk.
- Exploit Likelihood: High, due to the critical nature of the vulnerability.
- Mitigation Rate: Rapid patching is recommended to prevent exploitation.
By following these steps, users can both understand and mitigate the risks associated with CVE-2025-21786.
References:
Reported By: https://nvd.nist.gov/vuln/detail/CVE-2025-21786
Extra Source Hub:
Undercode
