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.

bash

# 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