CVE-2021-42739: Linux Kernel Buffer Overflow Vulnerability

CVE-2021-42739 Overview

CVE-2021-42739 is a buffer overflow vulnerability in the Linux kernel's FireWire subsystem affecting versions through 5.14.13. The vulnerability exists in the drivers/media/firewire/firedtv-avc.c and drivers/media/firewire/firedtv-ci.c driver files, where the avc_ca_pmt function mishandles bounds checking. This flaw allows a local attacker with elevated privileges to potentially corrupt memory, leading to system compromise.

Critical Impact

A local attacker with high privileges can exploit this buffer overflow to achieve high-impact compromise of confidentiality, integrity, and availability on affected Linux systems and dependent products including Oracle Communications and StarWind solutions.

Affected Products

• Linux Kernel (through version 5.14.13)
• Fedora 33, 34, and 35
• Debian Linux 9.0
• StarWind SAN & NAS v8r12
• StarWind Virtual SAN v8r13 (build 14338)
• Oracle Communications Cloud Native Core Binding Support Function 22.1.3
• Oracle Communications Cloud Native Core Network Exposure Function 22.1.1
• Oracle Communications Cloud Native Core Policy 22.2.0

Discovery Timeline

• 2021-10-20 - CVE-2021-42739 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-42739

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), indicating that the affected code writes data past the end of the intended buffer. The flaw resides in the FireWire Digital TV (firedtv) driver subsystem, specifically within the AVC (Audio/Video Control) and CI (Common Interface) handling components.

The avc_ca_pmt function, which processes Conditional Access Program Map Table operations, fails to properly validate the size of incoming data before copying it into fixed-size buffers. This improper bounds checking creates a condition where an attacker can supply malformed or oversized input that overwrites adjacent memory regions.

The exploitation requires local access to the system and high privileges, but successful exploitation can result in arbitrary code execution in kernel context, complete system compromise, or denial of service through system crashes.

Root Cause

The root cause of this vulnerability is missing or inadequate bounds checking in the avc_ca_pmt function within the FireWire media drivers. The code fails to verify that the length of data being processed does not exceed the capacity of the destination buffer before performing memory operations. This is a classic case of improper input validation leading to an out-of-bounds write condition.

Attack Vector

The attack vector is local, requiring the attacker to have authenticated access to the target system with high privileges. The attacker would need to interact with the FireWire subsystem, potentially through crafted ioctl calls or by manipulating data structures passed to the vulnerable driver functions. While the attack complexity is low once access is obtained, the high privilege requirement significantly limits the practical exploitability of this vulnerability.

The vulnerability mechanism involves sending specially crafted input to the avc_ca_pmt function that exceeds expected buffer boundaries. When the function processes this input without proper validation, the overflow corrupts adjacent memory, potentially allowing the attacker to overwrite critical kernel data structures or inject malicious code into kernel space. For detailed technical analysis, refer to the Linux Kernel Commit and the OSS Security Mailing List discussion.

Detection Methods for CVE-2021-42739

Indicators of Compromise

• Unexpected kernel crashes or system instability related to the FireWire subsystem
• Unusual activity or error messages in kernel logs referencing firedtv-avc or firedtv-ci modules
• Evidence of privilege escalation attempts from high-privileged accounts targeting kernel drivers
• Memory corruption artifacts in kernel memory analysis

Detection Strategies

• Monitor system logs for kernel oops, panics, or warnings related to FireWire media drivers
• Implement kernel integrity monitoring to detect unauthorized modifications to kernel memory
• Deploy SentinelOne agents configured to monitor for suspicious kernel-level activity and buffer overflow exploitation patterns
• Utilize security scanning tools to identify systems running vulnerable Linux kernel versions

Monitoring Recommendations

• Enable comprehensive kernel audit logging to capture interactions with FireWire device drivers
• Implement file integrity monitoring on kernel modules in /lib/modules/ related to FireWire functionality
• Configure alerting for any unexpected loading or modification of firedtv-avc and firedtv-ci kernel modules
• Regularly audit systems to ensure vulnerable kernel versions have been patched

How to Mitigate CVE-2021-42739

Immediate Actions Required

• Update the Linux kernel to a version that includes the security patch (commit 35d2969ea3c7d32aee78066b1f3cf61a0d935a4e)
• Apply vendor-specific patches from your Linux distribution (Fedora, Debian, etc.)
• Review and update Oracle Communications and StarWind products to patched versions as per their security advisories
• If immediate patching is not possible, consider disabling or blacklisting the affected FireWire media modules

Patch Information

The vulnerability has been addressed in upstream Linux kernel commits. Organizations should apply patches available from their respective Linux distribution vendors. Key resources include:

• Linux Kernel Commit - Upstream fix
• Red Hat Bug Report - Red Hat tracking
• Oracle Security Alert July 2022 - Oracle product patches
• StarWind Security Advisory - StarWind product patches

Workarounds

• Blacklist the firedtv kernel module if FireWire Digital TV functionality is not required
• Restrict access to systems to minimize the pool of potential attackers with high privileges
• Implement strict privilege separation to limit the number of users with elevated kernel-level access
• Deploy additional kernel hardening measures such as KASLR, SMEP, and SMAP to increase exploitation difficulty

# Blacklist vulnerable FireWire modules if not needed
echo "blacklist firedtv" | sudo tee /etc/modprobe.d/disable-firedtv.conf
echo "blacklist firewire-core" | sudo tee -a /etc/modprobe.d/disable-firedtv.conf

# Verify kernel version and check if patched
uname -r

# Check if vulnerable modules are loaded
lsmod | grep -i fire