CVE-2020-36766: Linux Kernel Information Disclosure Flaw

CVE-2020-36766 Overview

CVE-2020-36766 is a Memory Information Disclosure vulnerability discovered in the Linux kernel before version 5.8.6. The vulnerability exists in drivers/media/cec/core/cec-api.c, where one byte of kernel memory can leak to unprivileged users on specific hardware. This issue occurs due to directly assigning log_addrs with a hole in the struct, which can expose uninitialized kernel memory when copied to user space.

Critical Impact

Local attackers with low privileges can exploit this vulnerability to leak kernel memory information, potentially exposing sensitive data from kernel address space on systems with CEC (Consumer Electronics Control) hardware.

Affected Products

• Linux Kernel versions prior to 5.8.6
• Systems with CEC (Consumer Electronics Control) hardware support
• linux linux_kernel

Discovery Timeline

• 2023-09-18 - CVE CVE-2020-36766 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-36766

Vulnerability Analysis

This vulnerability stems from an improper memory handling practice in the Linux kernel's CEC (Consumer Electronics Control) API implementation. When the kernel copies the cec_log_addrs structure from kernel space to user space, it uses direct struct assignment. The cec_log_addrs structure contains a padding hole at the end due to memory alignment requirements. Using direct assignment instead of memcpy() can leave this hole uninitialized, containing stale kernel memory data.

When copy_to_user() is subsequently called to transfer this data to user space, the uninitialized byte in the struct hole gets leaked to the unprivileged user. While limited to one byte per operation, repeated exploitation could potentially reveal meaningful kernel memory information over time.

Root Cause

The root cause lies in the C language's handling of struct assignment. When assigning one struct to another using the = operator, compilers may optimize the operation and ignore padding bytes within the structure. The cec_log_addrs structure has alignment padding at the end, and direct assignment leaves this padding uninitialized. The correct approach is to use memcpy() which copies the entire memory region including padding bytes, ensuring no uninitialized data remains.

Attack Vector

This vulnerability requires local access and low privileges to exploit. An attacker needs:

1. Local access to the target system
2. Systems equipped with CEC-capable hardware (typically found in multimedia devices and displays)
3. Access to the CEC device interface
4. Ability to repeatedly query the CEC API to accumulate leaked bytes

The attack vector is local (AV:L) with low complexity (AC:L) and requires low privileges (PR:L), affecting confidentiality only with no impact on integrity or availability.

 	struct cec_log_addrs log_addrs;
 
 	mutex_lock(&adap->lock);
-	log_addrs = adap->log_addrs;
+	/*
+	 * We use memcpy here instead of assignment since there is a
+	 * hole at the end of struct cec_log_addrs that an assignment
+	 * might ignore. So when we do copy_to_user() we could leak
+	 * one byte of memory.
+	 */
+	memcpy(&log_addrs, &adap->log_addrs, sizeof(log_addrs));
 	if (!adap->is_configured)
 		memset(log_addrs.log_addr, CEC_LOG_ADDR_INVALID,
 		       sizeof(log_addrs.log_addr));

Source: GitHub Linux Commit 6c42227

Detection Methods for CVE-2020-36766

Indicators of Compromise

• Unusual access patterns to CEC device files (e.g., /dev/cec*)
• High frequency of ioctl calls to CEC devices from unprivileged processes
• Processes attempting to access CEC hardware without legitimate multimedia purposes

Detection Strategies

• Monitor system calls related to CEC device access using audit subsystem or eBPF-based tracing
• Implement kernel auditing for ioctl operations targeting CEC devices
• Use SentinelOne's behavioral AI to detect anomalous access patterns to kernel device interfaces

Monitoring Recommendations

• Enable Linux audit rules for CEC device file access
• Deploy kernel-level monitoring for memory disclosure attempts
• Review system logs for processes accessing /dev/cec* devices unexpectedly

How to Mitigate CVE-2020-36766

Immediate Actions Required

• Upgrade the Linux kernel to version 5.8.6 or later immediately
• If immediate patching is not possible, restrict access to CEC device files
• Audit systems for CEC hardware presence and disable if not required
• Review and restrict user permissions for multimedia device access

Patch Information

The vulnerability was addressed in Linux kernel version 5.8.6. The fix replaces the direct struct assignment with memcpy() to ensure all bytes including padding are properly initialized before copying to user space. The patch is available through the official kernel repositories.

For detailed patch information, refer to:

• Linux Kernel ChangeLog 5.8.6
• GitHub Linux Commit 6c42227

Workarounds

• Remove or blacklist the CEC kernel module (cec.ko) if CEC functionality is not required
• Restrict access to CEC device nodes using filesystem permissions (chmod 600 /dev/cec*)
• Use SELinux or AppArmor policies to limit CEC device access to authorized applications only

# Configuration example
# Disable CEC kernel module loading
echo "blacklist cec" >> /etc/modprobe.d/blacklist-cec.conf

# Restrict CEC device permissions
chmod 600 /dev/cec*

# Remove existing CEC module if loaded
modprobe -r cec