CVE-2021-41073: Linux Kernel Privilege Escalation Flaw

CVE-2021-41073 Overview

CVE-2021-41073 is a Use After Free vulnerability in the loop_rw_iter function within fs/io_uring.c in the Linux kernel versions 5.10 through 5.14.6. This vulnerability allows local users to escalate privileges by exploiting the IORING_OP_PROVIDE_BUFFERS operation to trigger an improper free of a kernel buffer. The exploitation technique has been demonstrated using /proc/<pid>/maps for information disclosure to aid in exploitation.

Critical Impact

Local privilege escalation allowing unprivileged users to gain root access on vulnerable Linux systems through kernel memory corruption in the io_uring subsystem.

Affected Products

• Linux Kernel versions 5.10 through 5.14.6
• Debian Linux 10.0
• Fedora 33 and 34
• NetApp Cloud Backup
• NetApp SolidFire Baseboard Management Controller
• NetApp H-Series (H300S, H500S, H700S, H300E, H500E, H700E, H410S, H410C) firmware

Discovery Timeline

• September 19, 2021 - CVE-2021-41073 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-41073

Vulnerability Analysis

The vulnerability resides in the io_uring subsystem, a modern asynchronous I/O interface introduced in Linux kernel 5.1. The loop_rw_iter function in fs/io_uring.c contains a flaw in how it handles buffer management during certain I/O operations. When processing IORING_OP_PROVIDE_BUFFERS requests, the kernel can be tricked into freeing a buffer that is still referenced elsewhere, creating a use-after-free condition.

The io_uring interface is designed for high-performance asynchronous I/O and is accessible to unprivileged users by default on many Linux distributions. This makes the vulnerability particularly dangerous as it provides a reliable local privilege escalation vector without requiring any special permissions.

Root Cause

The root cause is classified as CWE-763 (Release of Invalid Pointer or Reference). The vulnerability occurs due to improper lifetime management of kernel buffers within the io_uring subsystem. When the IORING_OP_PROVIDE_BUFFERS operation is used, the code fails to properly track buffer ownership, leading to a scenario where a kernel buffer can be freed while still being referenced by other kernel structures.

This type of memory corruption bug in the kernel is particularly severe because:

• The freed memory can be reallocated for attacker-controlled data
• Kernel memory corruption can bypass all userspace security boundaries
• The io_uring interface provides precise control over buffer operations

Attack Vector

The attack requires local access to the system with the ability to execute code as an unprivileged user. The attacker can leverage the io_uring interface to:

1. Create an io_uring instance and register buffers using IORING_OP_PROVIDE_BUFFERS
2. Trigger the vulnerable code path in loop_rw_iter to cause improper buffer release
3. Leverage the use-after-free condition to achieve controlled kernel memory corruption
4. Use /proc/<pid>/maps to leak kernel address space layout information to defeat KASLR
5. Achieve privilege escalation to root by corrupting kernel credential structures

The exploitation technique demonstrates that attackers can reliably exploit this vulnerability to gain full root privileges on affected systems.

Detection Methods for CVE-2021-41073

Indicators of Compromise

• Unusual io_uring system call activity from unprivileged processes
• Processes accessing /proc/self/maps or other processes' memory maps in suspicious contexts
• Unexpected privilege changes or new root processes spawned from unprivileged user accounts
• Kernel crash logs or OOPS messages related to io_uring or memory corruption

Detection Strategies

• Monitor for anomalous io_uring usage patterns, particularly IORING_OP_PROVIDE_BUFFERS operations followed by read/write operations
• Implement kernel-level auditing to track io_uring system calls from untrusted processes
• Deploy endpoint detection solutions that can identify privilege escalation attempts and kernel exploitation patterns
• Use seccomp-bpf filters to restrict io_uring access for high-risk applications

Monitoring Recommendations

• Enable kernel audit logging for io_uring-related syscalls (io_uring_setup, io_uring_enter, io_uring_register)
• Monitor for processes that suddenly gain elevated capabilities or change effective UID to 0
• Implement file integrity monitoring on critical system binaries that could indicate post-exploitation activity
• Review system logs for kernel OOPS or panic events that may indicate failed exploitation attempts

How to Mitigate CVE-2021-41073

Immediate Actions Required

• Update the Linux kernel to version 5.14.7 or later which contains the fix for this vulnerability
• For systems that cannot be immediately updated, consider disabling io_uring functionality using the io_uring_disabled sysctl parameter
• Apply vendor-specific patches from Debian, Fedora, or NetApp as applicable to your environment
• Limit local access to systems running vulnerable kernel versions

Patch Information

The vulnerability was addressed in the Linux kernel through commit 16c8d2df7ec0eed31b7d3b61cb13206a7fb930cc. This patch is available in the Linux Kernel Git Repository.

Distribution-specific patches are available:

• Debian: DSA-4978 Security Advisory
• Fedora: Updates available through the Fedora Package Announcements
• NetApp: Security Advisory NTAP-20211014-0003

Workarounds

• Disable io_uring system-wide by setting kernel.io_uring_disabled=2 in sysctl configuration (requires kernel 5.12.4+ for this sysctl)
• Use seccomp profiles to block io_uring syscalls for untrusted applications
• Restrict local user access on systems where patching is not immediately feasible
• Consider deploying containers with io_uring explicitly disabled in their seccomp profiles

# Disable io_uring system-wide (kernel 5.12.4+)
echo "kernel.io_uring_disabled=2" >> /etc/sysctl.d/99-disable-io_uring.conf
sysctl -p /etc/sysctl.d/99-disable-io_uring.conf

# Verify io_uring is disabled
cat /proc/sys/kernel/io_uring_disabled
# Output should be: 2