CVE-2020-25669: Linux Kernel Use-After-Free Vulnerability

CVE-2020-25669 Overview

CVE-2020-25669 is a Use After Free vulnerability discovered in the Linux Kernel's Sun keyboard driver (sunkbd). The vulnerability occurs when the function sunkbd_reinit is scheduled by sunkbd_interrupt before the sunkbd structure is freed. Although the dangling pointer is set to NULL in sunkbd_disconnect, an alias pointer remains in sunkbd_reinit, creating a Use After Free condition that can be exploited by a local attacker.

Critical Impact

Local attackers with low privileges can exploit this memory corruption vulnerability to potentially achieve code execution or cause system crashes, compromising system confidentiality, integrity, and availability.

Affected Products

• Linux Kernel (multiple versions)
• Debian Linux 9.0
• NetApp Cloud Backup
• NetApp SolidFire & HCI Management Node
• NetApp H300s, H500s, H700s, H300e, H500e, H700e, H410s, H410c (firmware and hardware)

Discovery Timeline

• November 5, 2020 - Vulnerability disclosed via Openwall OSS-Security mailing list
• November 20, 2020 - Additional details published on Openwall OSS-Security
• May 26, 2021 - CVE-2020-25669 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-25669

Vulnerability Analysis

This Use After Free vulnerability exists in the Linux Kernel's Sun keyboard input driver, specifically within the drivers/input/keyboard/sunkbd.c file. The core issue lies in a race condition between the interrupt handler and the disconnect path.

When a keyboard reset event occurs, sunkbd_interrupt schedules the sunkbd_reinit work item using schedule_work(). However, if sunkbd_disconnect is called while this work item is still pending, the sunkbd structure gets freed while sunkbd_reinit still holds a reference to it. Although sunkbd_disconnect sets the pointer to NULL, the work queue retains an alias pointer that references the now-freed memory.

An attacker with local access can exploit this timing window to trigger memory corruption, potentially leading to arbitrary code execution with kernel privileges or causing a denial of service through system crashes.

Root Cause

The root cause is improper synchronization between the keyboard interrupt handler (sunkbd_interrupt) and the disconnect handler (sunkbd_disconnect). The work item scheduled by the interrupt handler is not properly cancelled or guarded before the sunkbd structure is freed during device disconnection. This creates a classic Use After Free scenario where the sunkbd_reinit function operates on memory that has already been deallocated.

Attack Vector

This vulnerability requires local access to the system. An attacker must be able to interact with the Sun keyboard driver, typically by connecting or manipulating input devices. The attack exploits the race condition between scheduling the reinit work and the device disconnection process. Successful exploitation requires precise timing but does not require elevated privileges initially.

// Patch from drivers/input/keyboard/sunkbd.c
// Source: https://github.com/torvalds/linux/commit/77e70d351db7de07a46ac49b87a6c3c7a60fca7e

 	switch (data) {
 
 	case SUNKBD_RET_RESET:
-		schedule_work(&sunkbd->tq);
+		if (sunkbd->enabled)
+			schedule_work(&sunkbd->tq);
 		sunkbd->reset = -1;
 		break;

The fix introduces an enabled flag check before scheduling the work item. This ensures that sunkbd_reinit is only scheduled when the device is in a valid, connected state, preventing the Use After Free condition.

Detection Methods for CVE-2020-25669

Indicators of Compromise

• Unexpected kernel panics or crashes related to the sunkbd module or input subsystem
• Kernel log messages indicating memory corruption in drivers/input/keyboard/sunkbd.c
• Suspicious activity involving repeated connection/disconnection of input devices
• Stack traces referencing sunkbd_reinit or sunkbd_interrupt in kernel oops messages

Detection Strategies

• Monitor kernel logs (dmesg) for Use After Free warnings or memory corruption errors related to input drivers
• Implement kernel address sanitizer (KASAN) in development environments to detect memory safety violations
• Deploy endpoint detection solutions capable of monitoring kernel-level anomalies and unexpected driver behavior
• Audit system logs for unusual input device enumeration patterns that could indicate exploitation attempts

Monitoring Recommendations

• Enable kernel debugging options to capture detailed crash information if the vulnerability is triggered
• Configure centralized logging for kernel messages across affected Linux systems
• Monitor for unexpected system reboots or crashes on systems running vulnerable kernel versions
• Implement SentinelOne's kernel-level monitoring to detect anomalous behavior in the input subsystem

How to Mitigate CVE-2020-25669

Immediate Actions Required

• Update the Linux Kernel to a patched version that includes commit 77e70d351db7de07a46ac49b87a6c3c7a60fca7e
• Apply vendor-specific patches from Debian, NetApp, or your distribution's security updates
• Prioritize patching systems that may have physical access to Sun keyboards or SPARC hardware
• Review the NetApp Security Advisory for affected storage appliance firmware updates

Patch Information

The vulnerability has been patched in the upstream Linux Kernel. The fix is available in commit 77e70d351db7de07a46ac49b87a6c3c7a60fca7e. Debian has released security updates documented in their LTS announcements. Organizations using NetApp products should consult the NetApp security advisory for applicable firmware updates.

Workarounds

• If immediate patching is not possible, consider disabling or blacklisting the sunkbd module on systems where Sun keyboards are not required
• Restrict physical access to affected systems to minimize the attack surface
• Monitor affected systems closely for signs of exploitation until patches can be applied
• For virtualized environments, ensure guest kernel updates are applied in addition to host updates

# Blacklist the sunkbd module if not needed
echo "blacklist sunkbd" >> /etc/modprobe.d/blacklist-sunkbd.conf

# Verify the module is not loaded
lsmod | grep sunkbd

# If loaded, remove it (requires root privileges)
rmmod sunkbd