CVE-2021-3491: Linux Kernel io_uring RCE Vulnerability

CVE-2021-3491 Overview

CVE-2021-3491 is a heap overflow vulnerability in the Linux kernel's io_uring subsystem that allows local attackers to achieve arbitrary code execution with kernel privileges. The vulnerability exists in the PROVIDE_BUFFERS operation, where the MAX_RW_COUNT limit could be bypassed, leading to negative values being used in mem_rw when reading /proc/<PID>/mem. This memory corruption can be leveraged to create a heap overflow, ultimately enabling complete system compromise.

Critical Impact

Local privilege escalation to kernel-level code execution via heap overflow in Linux kernel io_uring subsystem, potentially leading to full system compromise.

Affected Products

• Linux Kernel versions 5.7-rc1 through 5.12.3
• Canonical Ubuntu Linux 20.04 LTS
• Canonical Ubuntu Linux 20.10
• Canonical Ubuntu Linux 21.04

Discovery Timeline

• 2021-06-04 - CVE-2021-3491 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3491

Vulnerability Analysis

The io_uring subsystem, introduced in Linux kernel 5.1, provides a high-performance asynchronous I/O interface. The vulnerability was introduced in commit ddf0322db79c ("io_uring: add IORING_OP_PROVIDE_BUFFERS") which was part of the v5.7-rc1 release cycle.

The core issue lies in insufficient input validation within the PROVIDE_BUFFERS operation. When a user supplies buffer lengths to the io_uring interface, the kernel fails to properly validate that these lengths do not exceed MAX_RW_COUNT. This validation gap allows attackers to supply specially crafted values that, when processed, result in integer overflow conditions that produce negative values in the mem_rw function.

When these negative values are subsequently used for memory operations while reading /proc/<PID>/mem, the kernel's memory allocation and access routines behave unexpectedly. This creates a heap overflow condition where data can be written beyond the intended buffer boundaries, corrupting adjacent kernel heap memory structures.

Root Cause

The root cause is an improper calculation of buffer size (CWE-131) combined with an out-of-bounds write condition (CWE-787) in the io_uring PROVIDE_BUFFERS operation. The kernel code did not truncate or validate buffer lengths against MAX_RW_COUNT before using them in memory operations, allowing integer wraparound that results in heap corruption.

Attack Vector

The attack requires local access to the system with the ability to invoke io_uring syscalls. An attacker can exploit this vulnerability by:

1. Opening an io_uring instance using the io_uring_setup() syscall
2. Submitting a PROVIDE_BUFFERS operation with a maliciously crafted length value designed to bypass MAX_RW_COUNT validation
3. Triggering the vulnerable code path through /proc/<PID>/mem read operations
4. Leveraging the resulting heap overflow to corrupt kernel data structures
5. Achieving arbitrary code execution in kernel context, effectively gaining root privileges

The exploitation occurs entirely locally and does not require user interaction, though it does require the attacker to have unprivileged local access to the target system.

Detection Methods for CVE-2021-3491

Indicators of Compromise

• Unusual io_uring syscall activity with abnormally large buffer size parameters
• Kernel panic or oops messages related to io_uring or memory subsystems
• Unexpected privilege escalation events on systems running vulnerable kernel versions
• Anomalous access patterns to /proc/<PID>/mem following io_uring operations

Detection Strategies

• Monitor for processes making io_uring syscalls with unusually large buffer specifications using audit frameworks like auditd
• Deploy kernel integrity monitoring to detect unauthorized kernel memory modifications
• Implement behavioral analysis to identify local privilege escalation attempts following io_uring activity
• Use SentinelOne's kernel-level protection to detect and block heap overflow exploitation attempts

Monitoring Recommendations

• Enable kernel auditing for io_uring related syscalls (io_uring_setup, io_uring_enter, io_uring_register)
• Configure alerts for kernel panic events that reference io_uring or provide_buffers in stack traces
• Monitor for processes attempting to read /proc/self/mem or /proc/<PID>/mem after io_uring operations
• Review system logs for signs of privilege escalation or unexpected root access

How to Mitigate CVE-2021-3491

Immediate Actions Required

• Update to patched kernel versions: 5.12.4, 5.11.21, or 5.10.37 (or later)
• Apply distribution-specific security updates from Ubuntu (USN-4949-1, USN-4950-1) or other vendors
• If immediate patching is not possible, consider disabling io_uring functionality via kernel parameters
• Restrict local system access to trusted users only until patching is complete

Patch Information

The vulnerability was addressed in commit d1f82808877b ("io_uring: truncate lengths larger than MAX_RW_COUNT on provide buffers") which was included in Linux kernel v5.13-rc1 and backported to stable releases v5.12.4, v5.11.21, and v5.10.37.

For detailed patch information, refer to:

• Linux Kernel Commit Update
• Ubuntu Security Notice USN-4949-1
• Ubuntu Security Notice USN-4950-1
• Zero Day Initiative Advisory ZDI-21-589

Workarounds

• Disable io_uring by adding io_uring.enable=0 to kernel boot parameters if the feature is not required
• Use kernel lockdown mode to prevent loading of unsigned kernel modules and restrict debugging interfaces
• Implement mandatory access control (SELinux/AppArmor) policies to restrict io_uring syscall access to specific processes
• Deploy container isolation to limit the blast radius of potential exploitation

# Disable io_uring via kernel boot parameter
# Add to /etc/default/grub GRUB_CMDLINE_LINUX line:
GRUB_CMDLINE_LINUX="io_uring.enable=0"

# Then update grub configuration
sudo update-grub
sudo reboot