CVE-2021-3411: Linux Kernel Privilege Escalation Flaw

CVE-2021-3411 Overview

CVE-2021-3411 is a memory access violation in the Linux kernel affecting versions prior to 5.10. The flaw occurs while detecting padding of an int3 instruction in the linking state, a code path used by the kernel's live patching and tracing infrastructure. A local attacker with elevated privileges can trigger improper memory access, impacting data confidentiality, integrity, and system availability. The vulnerability is classified under [CWE-94] (Improper Control of Generation of Code) and affects both upstream Linux kernel builds and downstream distributions such as Red Hat Enterprise Linux 8.

Critical Impact
A local user with high privileges can exploit improper int3 padding detection in the Linux kernel to compromise data confidentiality, integrity, and availability on the host.

Affected Products

Linux Kernel versions prior to 5.10
Red Hat Enterprise Linux 8.0
Distributions shipping kernels that include the affected int3 linking-state padding detection code

Discovery Timeline

2021-03-09 - CVE-2021-3411 published to the National Vulnerability Database
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3411

Vulnerability Analysis

The Linux kernel uses the int3 instruction (opcode 0xCC) as a software breakpoint primitive in features such as kprobes, ftrace, and live patching. When the kernel transitions code into a linked state, it must validate padding around the inserted int3 to ensure instruction stream consistency. The padding detection logic prior to kernel 5.10 contains a memory access violation that reads or operates on memory outside the intended bounds.

Because the affected code runs in kernel context, any out-of-bounds access can corrupt adjacent structures or leak kernel memory. The flaw requires local access and high privileges, which limits the attack surface to scenarios where an attacker already controls a privileged user account or compromised service.

For deeper technical analysis, see the PI3 Blog Post and the Red Hat Bug Report.

Root Cause

The root cause is improper bounds handling in the kernel routine that detects int3 padding during the linking state of dynamic code modification. The function operates on instruction memory without sufficient validation, leading to a memory access violation that the kernel does not safely contain.

Attack Vector

Exploitation requires local access to the system with high privileges. An attacker invokes kernel paths that exercise the dynamic patching or tracing subsystems, triggering the faulty padding detection. Successful exploitation can corrupt kernel memory, disclose sensitive kernel data, or destabilize the system into a denial-of-service state.

No public proof-of-concept code is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. The mechanism is described in the referenced advisories; readers should consult those sources rather than synthetic exploit samples.

Detection Methods for CVE-2021-3411

Indicators of Compromise

Unexpected kernel oops or panic entries in dmesg referencing int3, text_poke, or ftrace code paths
Crashes during loading or unloading of kernel modules and live patches
Anomalous kernel memory access faults logged by hardware error reporting on systems running kernels older than 5.10

Detection Strategies

Inventory running kernel versions across Linux hosts and flag any kernel earlier than 5.10 or any RHEL 8 build that has not received the corresponding errata
Correlate kernel crash dumps with use of kprobes, ftrace, and live patching subsystems to identify systems exercising the vulnerable code path
Monitor privileged process activity that loads kernel modules or modifies kernel text in production environments

Monitoring Recommendations

Centralize kernel logs and crash reports into a SIEM or data lake for longitudinal analysis of kernel faults
Alert on repeated BUG: or general protection fault messages tied to kernel patching subsystems
Track package update status for kernel and kernel-rt across the fleet to ensure patched builds are deployed

How to Mitigate CVE-2021-3411

Immediate Actions Required

Upgrade affected hosts to Linux kernel 5.10 or later, or apply the vendor-supplied backport for your distribution
For Red Hat Enterprise Linux 8, install the kernel errata referenced in the Red Hat Bug Report
Restrict local privileged access on systems that cannot be patched immediately

Patch Information

The fix is included in Linux kernel 5.10 and later. Downstream distributions including Red Hat Enterprise Linux 8 ship the correction through their respective kernel security updates. Apply vendor errata using the standard package management workflow, then reboot to load the patched kernel.

Workarounds

Limit the number of users with CAP_SYS_ADMIN or other high-privilege capabilities that can trigger kernel patching paths
Disable optional kernel features that exercise the affected code, such as live kernel patching, where operationally feasible
Enforce kernel module signing and restrict module loading to reduce the chance of attacker-controlled code reaching the vulnerable path

bash

# Verify the running kernel version and update on RHEL 8
uname -r
sudo dnf update kernel
sudo reboot

# Verify on Debian/Ubuntu systems
uname -r
sudo apt update && sudo apt upgrade linux-image-$(uname -r | cut -d- -f3-)
sudo reboot