CVE-2020-11725: Linux Kernel Privilege Escalation Flaw

CVE-2020-11725 Overview

CVE-2020-11725 is a vulnerability in snd_ctl_elem_add within sound/core/control.c in the Linux kernel through version 5.6.3. The vulnerability involves an assignment count=info->owner which subsequently affects a private_size*count multiplication, potentially leading to integer overflow conditions with unspecified side effects.

It is important to note that Linux kernel engineers have disputed this finding. According to their assessment, this could only be relevant if new callers were added that were unfamiliar with the intentional misuse of the info->owner field to represent data unrelated to the "owner" concept. The existing callers, SNDRV_CTL_IOCTL_ELEM_ADD and SNDRV_CTL_IOCTL_ELEM_REPLACE, have been designed to misuse the info->owner field in a safe manner.

Critical Impact

Local attackers with low privileges could potentially exploit this integer overflow in the ALSA sound subsystem to achieve high impact on confidentiality, integrity, and availability of the system.

Affected Products

• Linux Kernel through version 5.6.3
• All Linux distributions using affected kernel versions
• Systems with ALSA sound subsystem enabled

Discovery Timeline

• April 12, 2020 - CVE-2020-11725 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-11725

Vulnerability Analysis

The vulnerability exists in the Advanced Linux Sound Architecture (ALSA) subsystem, specifically within the snd_ctl_elem_add function located in sound/core/control.c. The issue centers around the unconventional use of the info->owner field, which is being repurposed to store count information rather than ownership data.

When a user-space application invokes the SNDRV_CTL_IOCTL_ELEM_ADD or SNDRV_CTL_IOCTL_ELEM_REPLACE ioctl calls, the kernel processes control element additions through snd_ctl_elem_add. The function assigns count=info->owner, and this count value is subsequently used in a multiplication operation with private_size. If an attacker could introduce a new code path that misunderstands this field usage, the multiplication could potentially overflow, leading to incorrect memory allocation sizes.

The attack requires local access to the system and low-privilege user credentials to interact with the ALSA subsystem through ioctl calls.

Root Cause

The root cause stems from an unconventional design pattern where the info->owner field is repurposed to store count data rather than actual ownership information. This semantic mismatch between the field name and its actual purpose creates a potential trap for future developers who might add new callers without understanding this intentional misuse. The existing callers were designed with this behavior in mind, but the pattern violates the principle of least astonishment and could lead to vulnerabilities if new code is added without proper documentation review.

Attack Vector

The attack vector requires local access to the Linux system with low privileges. An attacker would need to:

1. Gain local access to a system running an affected Linux kernel version
2. Interact with the ALSA sound control interface through ioctl system calls
3. Craft specific input values that could trigger the integer overflow condition in the private_size*count multiplication

The vulnerability mechanism involves the following flow: When processing control element operations, the kernel function retrieves a count value from what should logically be an owner field. This count is then multiplied with a private size value. If the multiplication overflows, it could result in undersized buffer allocations, potentially leading to heap-based memory corruption issues.

For detailed technical analysis, refer to the GitHub Linux Code Reference showing the affected code section.

Detection Methods for CVE-2020-11725

Indicators of Compromise

• Unusual activity involving ALSA ioctl calls, specifically SNDRV_CTL_IOCTL_ELEM_ADD and SNDRV_CTL_IOCTL_ELEM_REPLACE
• Kernel memory corruption indicators or unexpected kernel panics related to the sound subsystem
• Anomalous sound control element creation attempts in audit logs
• Process crashes or unexpected behavior in applications interacting with sound control interfaces

Detection Strategies

• Monitor system calls related to ALSA sound control operations using tools like auditd or eBPF-based monitoring
• Implement kernel integrity monitoring to detect potential memory corruption in the ALSA subsystem
• Deploy SentinelOne Singularity Platform for real-time kernel-level threat detection and behavioral analysis
• Review kernel logs for ALSA-related errors or warnings that might indicate exploitation attempts

Monitoring Recommendations

• Enable audit rules for ioctl calls targeting /dev/snd/controlC* devices
• Configure system monitoring to alert on unusual patterns of sound control element creation
• Implement SentinelOne's automated threat detection to identify exploitation attempts at the kernel level
• Regularly review kernel dmesg output for anomalies in the sound subsystem

How to Mitigate CVE-2020-11725

Immediate Actions Required

• Assess your environment for systems running Linux kernel versions through 5.6.3
• Prioritize kernel updates on systems where local user access is less controlled
• Review any custom kernel modules or applications that interact with ALSA control interfaces
• Restrict local user access on critical systems where possible

Patch Information

Organizations should update to Linux kernel versions beyond 5.6.3 that include hardened handling of the ALSA control element operations. Consult your Linux distribution's security advisory channels for specific patched kernel versions:

• For enterprise distributions (RHEL, CentOS, Ubuntu LTS, SLES), apply the latest kernel security updates from your vendor
• Check the Kernel Mailing List Post for kernel developer discussions on this issue
• Verify kernel version after patching using uname -r command

Workarounds

• Restrict access to ALSA control devices (/dev/snd/controlC*) using filesystem permissions to limit which users can interact with the sound subsystem
• Implement mandatory access control policies using SELinux or AppArmor to restrict ioctl operations on sound devices
• Consider disabling the ALSA sound subsystem on servers or systems where audio functionality is not required
• Use container isolation with restricted device access to limit potential attack surface

# Configuration example - Restrict ALSA control device access
# Add only trusted users to the 'audio' group
chmod 660 /dev/snd/controlC*
chown root:audio /dev/snd/controlC*

# Verify current permissions
ls -la /dev/snd/controlC*

# Optional: Disable ALSA module loading if audio not required
echo "blacklist snd" >> /etc/modprobe.d/blacklist-audio.conf
echo "blacklist snd_hda_intel" >> /etc/modprobe.d/blacklist-audio.conf