CVE-2020-25669 Overview
CVE-2020-25669 is a use-after-free (UAF) vulnerability in the Linux kernel's Sun keyboard driver (drivers/input/keyboard/sunkbd.c). The flaw exists because the sunkbd_reinit function can be scheduled by sunkbd_interrupt before the sunkbd structure is freed. Although sunkbd_disconnect sets the dangling pointer to NULL, an alias remains in sunkbd_reinit, leading to memory corruption [CWE-416].
The vulnerability affects multiple Linux kernel versions, Debian Linux distributions, and several NetApp HCI and SolidFire products that ship with affected kernels. Successful exploitation requires local access with low privileges.
Critical Impact
Local attackers with low privileges can trigger memory corruption in the kernel, potentially leading to privilege escalation, denial of service, or arbitrary code execution within kernel context.
Affected Products
- Linux Kernel (multiple versions)
- Debian Linux 9.0
- NetApp Cloud Backup, SolidFire & HCI Management Node, SolidFire Baseboard Management Controller, and H-series firmware (H300S, H500S, H700S, H300E, H500E, H700E, H410S, H410C)
Discovery Timeline
- 2021-05-26 - CVE-2020-25669 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2020-25669
Vulnerability Analysis
The vulnerability resides in the Sun keyboard input driver, which handles legacy Sun Microsystems keyboards connected via serial ports. The driver uses a workqueue mechanism to handle keyboard reset events asynchronously. When the keyboard sends a reset signal (SUNKBD_RET_RESET), sunkbd_interrupt schedules sunkbd_reinit to run as deferred work.
The race condition arises during driver teardown. If sunkbd_disconnect is called while a reinitialization work item is pending or executing, the sunkbd structure can be freed while sunkbd_reinit still holds a reference to it. The result is a classic use-after-free condition in kernel memory.
Root Cause
The root cause is improper lifecycle management of the sunkbd object between the interrupt handler and the deferred workqueue. The original code unconditionally scheduled work in response to a reset event without checking whether the driver was in a valid, enabled state. Setting the dangling pointer to NULL in sunkbd_disconnect was insufficient because sunkbd_reinit retained an aliased pointer that bypassed the nullification.
Attack Vector
Exploitation requires local access and the ability to interact with the affected input device. An attacker would need to trigger the disconnect path while a reset event is being processed, creating the race window. The vulnerability impacts confidentiality, integrity, and availability of the kernel.
switch (data) {
case SUNKBD_RET_RESET:
- schedule_work(&sunkbd->tq);
+ if (sunkbd->enabled)
+ schedule_work(&sunkbd->tq);
sunkbd->reset = -1;
break;
Source: GitHub Linux Commit 77e70d3 — The patch adds an enabled flag check before scheduling the reinit work, ensuring no new work is queued after the driver begins teardown.
Detection Methods for CVE-2020-25669
Indicators of Compromise
- Unexpected kernel oops or panic messages referencing sunkbd_reinit or workqueue execution in kernel logs (dmesg, /var/log/kern.log).
- KASAN (Kernel Address Sanitizer) reports flagging use-after-free in drivers/input/keyboard/sunkbd.c on instrumented kernels.
- Anomalous local user processes interacting with /dev/input/ serial keyboard devices on systems that do not normally use Sun keyboards.
Detection Strategies
- Inventory running kernel versions across Linux hosts and compare against the fixed versions published in the Debian LTS Advisory and NetApp Security Advisory.
- Monitor kernel ring buffer output for crashes originating from input subsystem workqueues.
- Audit loaded kernel modules with lsmod | grep sunkbd to identify systems where the vulnerable driver is active.
Monitoring Recommendations
- Forward kernel logs to a centralized logging or SIEM platform and alert on kernel oops, BUG, or KASAN events.
- Track local privilege escalation indicators such as unexpected processes spawning with elevated UIDs following kernel crashes.
- Apply endpoint detection and response (EDR) coverage on Linux hosts to flag post-exploitation behavior such as suspicious child processes from input-handling services.
How to Mitigate CVE-2020-25669
Immediate Actions Required
- Apply the upstream kernel patch from commit 77e70d351db7 or upgrade to a distribution kernel that includes the fix.
- Debian users should update to the patched packages described in the Debian LTS announcements.
- NetApp customers should review the NetApp Security Advisory NTAP-20210702-0006 for product-specific remediation guidance.
Patch Information
The fix is implemented in drivers/input/keyboard/sunkbd.c and adds an enabled state check before scheduling reinit work. The patch is available in the mainline Linux kernel via commit 77e70d351db7de07a46ac49b87a6c3c7a60fca7e. Distribution backports are available for Debian via the Debian Long Term Support security advisories.
Workarounds
- Blacklist the sunkbd kernel module on systems that do not require Sun keyboard support to eliminate the vulnerable code path.
- Restrict local user access on multi-user systems until the patched kernel is deployed.
- Disable physical or virtual access to serial ports that could attach a Sun-compatible keyboard device on production hosts.
# Blacklist the sunkbd module to prevent loading
echo "blacklist sunkbd" | sudo tee /etc/modprobe.d/blacklist-sunkbd.conf
sudo update-initramfs -u
# Verify the module is not loaded
lsmod | grep sunkbd
# Apply patched kernel (Debian example)
sudo apt update && sudo apt upgrade linux-image-$(uname -r)
sudo reboot
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.