CVE-2021-42739: Linux Kernel Buffer Overflow Vulnerability

CVE-2021-42739 Overview

CVE-2021-42739 is a buffer overflow vulnerability in the FireWire subsystem of the Linux kernel through version 5.14.13. The flaw resides in drivers/media/firewire/firedtv-avc.c and drivers/media/firewire/firedtv-ci.c, where the avc_ca_pmt function mishandles bounds checking. A local attacker with high privileges can trigger an out-of-bounds write, leading to memory corruption, denial of service, or local code execution in kernel context. The issue is classified under [CWE-787] Out-of-Bounds Write and affects multiple downstream Linux distributions and embedded products.

Critical Impact

Successful exploitation enables kernel-mode memory corruption, potentially resulting in privilege escalation, kernel panic, or arbitrary code execution on affected Linux systems.

Affected Products

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

The vulnerability exists in the FireDTV driver used by the Linux kernel FireWire subsystem to support digital television receivers over IEEE 1394. Within drivers/media/firewire/firedtv-avc.c, the avc_ca_pmt function processes Program Map Table (PMT) data passed from user space through the Common Interface (CI) ioctl handler in drivers/media/firewire/firedtv-ci.c. The function fails to properly validate the length of program information and elementary stream descriptor fields before copying data into fixed-size kernel buffers. An attacker supplying a crafted PMT structure can write past the bounds of the destination buffer, corrupting adjacent kernel memory.

Root Cause

The root cause is missing or insufficient bounds checking on attacker-controlled length fields within the PMT structure parsed by avc_ca_pmt. The function trusts the program_info_length and ES_info_length values supplied by user space without verifying they fit within the destination buffer, producing the out-of-bounds write condition [CWE-787].

Attack Vector

Exploitation requires local access and high privileges on the target system, along with the ability to issue ioctl calls to a FireDTV character device. The attacker submits a malformed PMT through the CA_SEND_MSG ioctl, causing the kernel to write attacker-controlled bytes beyond the allocated buffer. The corruption can lead to kernel panic or, with careful heap shaping, kernel-mode code execution. User interaction is not required and the impact is confined to the affected host.

No public proof-of-concept code has been released. See the Linux Kernel Commit Update and the OSS-Sec Security Discussion for the technical analysis.

Detection Methods for CVE-2021-42739

Indicators of Compromise

• Unexpected kernel oops or panic messages referencing avc_ca_pmt, firedtv-avc, or firedtv-ci in dmesg or /var/log/messages.
• Loading of the firedtv kernel module on systems that do not require FireWire DVB hardware.
• Process activity invoking ioctl calls against /dev/dvb/adapter*/ca0 from non-administrative contexts.

Detection Strategies

• Audit installed kernel versions and compare against the patched releases referenced in the Red Hat Bug Report.
• Monitor for unsigned or unexpected loading of the firedtv module using kernel auditing rules.
• Correlate kernel crash dumps with FireWire subsystem stack traces to identify exploitation attempts.

Monitoring Recommendations

• Enable auditd rules on init_module and finit_module syscalls to flag module loads.
• Centralize kernel logs and crash reports for retrospective analysis.
• Alert on local privilege escalation indicators such as unexpected UID transitions following kernel events.

How to Mitigate CVE-2021-42739

Immediate Actions Required

• Apply the upstream kernel patch referenced in commit 35d2969ea3c7d32aee78066b1f3cf61a0d935a4e or upgrade to a kernel version newer than 5.14.13.
• Install distribution updates for Fedora 33/34/35 and Debian 9, and apply Oracle's July 2022 Critical Patch Update for affected Communications Cloud Native Core products.
• Review the StarWind Security Advisory for SAN & NAS and Virtual SAN remediation.

Patch Information

The upstream fix adds proper bounds validation in avc_ca_pmt to prevent the out-of-bounds write. Refer to the Linux Kernel Commit Update for the authoritative patch and the Linux Media Mailing List Post for upstream discussion.

Workarounds

• Blacklist the firedtv kernel module on systems that do not require FireWire DVB functionality.
• Restrict access to /dev/dvb/* device nodes to trusted administrators using filesystem permissions.
• Disable automatic module loading for the FireWire subsystem where DVB hardware is not present.

# Blacklist the vulnerable firedtv module
echo "blacklist firedtv" | sudo tee /etc/modprobe.d/blacklist-firedtv.conf
sudo depmod -a
sudo rmmod firedtv 2>/dev/null || true

# Verify the module is no longer loaded
lsmod | grep firedtv