CVE-2021-33909: Linux Kernel Privilege Escalation Flaw

CVE-2021-33909 Overview

CVE-2021-33909, dubbed "Sequoia," is a critical integer overflow vulnerability in the Linux kernel's fs/seq_file.c component that affects kernel versions 3.16 through 5.13.x before 5.13.4. This vulnerability allows an unprivileged local user to achieve root privilege escalation through improper restriction of seq buffer allocations, leading to an out-of-bounds write condition.

The vulnerability exists in the sequential file interface implementation within the Linux kernel filesystem layer. When exploited, an attacker with local access can trigger an integer overflow during seq buffer allocation operations, ultimately corrupting kernel memory and gaining elevated privileges.

Critical Impact

Unprivileged local users can exploit this integer overflow vulnerability to escalate privileges to root, compromising complete system integrity across affected Linux kernel versions spanning nearly a decade of releases.

Affected Products

• Linux Kernel versions 3.16 through 5.13.x (before 5.13.4)
• Fedora 34
• Debian Linux 9.0 and 10.0
• NetApp HCI Management Node
• NetApp SolidFire
• Oracle Communications Session Border Controller (versions 8.2, 8.3, 8.4, 9.0)
• SonicWall SMA1000 Firmware

Discovery Timeline

• July 20, 2021 - CVE-2021-33909 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-33909

Vulnerability Analysis

The Sequoia vulnerability resides in the fs/seq_file.c file, which implements the sequential file interface used extensively throughout the Linux kernel for reading data from /proc and other virtual filesystems. The core issue stems from inadequate bounds checking when allocating seq buffers, allowing attackers to trigger an integer overflow condition.

When a user creates an extremely deep directory structure and then reads from certain /proc entries (such as /proc/self/mountinfo), the kernel attempts to allocate increasingly larger seq buffers to accommodate the lengthy path information. Without proper size validation, the allocation size can overflow, resulting in a smaller-than-expected buffer being allocated. Subsequent write operations then exceed the buffer boundaries, causing out-of-bounds memory corruption.

This memory corruption can be weaponized to overwrite critical kernel data structures, enabling privilege escalation from an unprivileged user to root. The vulnerability is particularly dangerous because it has existed in the kernel codebase since version 3.16, released in 2014, meaning it affects a substantial portion of deployed Linux systems.

Root Cause

The root cause is an integer overflow vulnerability (CWE-190) in the seq buffer allocation logic within fs/seq_file.c. The kernel failed to validate that requested buffer sizes did not exceed MAX_RW_COUNT before attempting allocation. When extremely large sizes are requested, the allocation size wraps around due to integer overflow, resulting in a buffer that is too small for the intended data. This allows subsequent operations to write beyond the allocated buffer boundaries, corrupting adjacent kernel memory.

Attack Vector

This is a local attack vector that requires an unprivileged user account on the target system. The attack methodology involves:

1. Creating an extremely deep nested directory structure (approximately 1GB total path length)
2. Mounting bind mounts to create extensive path entries
3. Reading from /proc/self/mountinfo or similar seq_file interfaces
4. Triggering the integer overflow during buffer allocation
5. Exploiting the out-of-bounds write to corrupt kernel memory structures
6. Achieving root privilege escalation

The attack requires local access but no special privileges, making it a significant threat to multi-tenant environments and systems where users have shell access.

// Security patch - seq_file: disallow extremely large seq buffer allocations
// Source: https://github.com/torvalds/linux/commit/8cae8cd89f05f6de223d63e6d15e31c8ba9cf53b
 
static void *seq_buf_alloc(unsigned long size)
{
+	if (unlikely(size > MAX_RW_COUNT))
+		return NULL;
+
	return kvmalloc(size, GFP_KERNEL_ACCOUNT);
}

The patch adds a simple bounds check to prevent allocation requests exceeding MAX_RW_COUNT, effectively mitigating the integer overflow condition before it can occur.

Detection Methods for CVE-2021-33909

Indicators of Compromise

• Unusually deep directory structures created on the filesystem (paths exceeding normal application requirements)
• Abnormal memory allocation patterns in kernel logs related to seq_file operations
• Processes accessing /proc/self/mountinfo with excessive read operations
• Evidence of bind mount abuse creating artificially long path entries
• Kernel crash dumps or oops messages referencing seq_file.c or seq buffer operations

Detection Strategies

• Monitor for the creation of extremely deep directory hierarchies that deviate from normal system behavior
• Implement file system auditing to detect rapid creation of nested directories by unprivileged users
• Deploy kernel-level monitoring to detect anomalous seq_file buffer allocation requests
• Utilize SentinelOne's behavioral AI to identify exploitation patterns associated with privilege escalation attempts

Monitoring Recommendations

• Enable comprehensive audit logging for filesystem operations, particularly mkdir and mount syscalls
• Monitor /proc/self/mountinfo access patterns for unusual frequency or volume
• Track kernel memory allocation events for signs of overflow conditions
• Review system logs for segmentation faults or kernel panics that may indicate exploitation attempts
• Deploy endpoint detection and response (EDR) solutions capable of detecting kernel-level exploitation

How to Mitigate CVE-2021-33909

Immediate Actions Required

• Update Linux kernel to version 5.13.4 or later, which contains the security fix
• Apply vendor-specific patches for affected distributions (Debian, Fedora, etc.)
• Restrict shell access for unprivileged users where possible until patching is complete
• Consider implementing kernel live patching solutions for environments requiring minimal downtime
• Review and audit systems for evidence of exploitation before patching

Patch Information

The vulnerability was addressed in Linux kernel version 5.13.4. The fix (commit 8cae8cd89f05f6de223d63e6d15e31c8ba9cf53b) adds a bounds check in the seq_buf_alloc() function to prevent allocation sizes exceeding MAX_RW_COUNT. Vendor-specific patches are available from multiple sources:

• Linux Kernel ChangeLog 5.13.4
• GitHub Commit on Linux
• Debian Security Advisory DSA-4941
• NetApp Security Advisory NTAP-20210819-0004
• SonicWall Vulnerability SNWLID-2022-0015

Workarounds

• Restrict unprivileged user access to shell accounts on affected systems
• Implement filesystem quotas to limit the number of inodes and directory depth per user
• Use Linux Security Modules (AppArmor, SELinux) to restrict access to sensitive /proc entries
• Consider deploying kernel live patches from vendors like Canonical or Red Hat for systems that cannot be immediately rebooted
• Monitor and limit bind mount operations through system policy configuration

# Verify current kernel version
uname -r

# Check if system is vulnerable (kernel < 5.13.4)
# Update kernel on Debian/Ubuntu
sudo apt update && sudo apt upgrade linux-image-generic

# Update kernel on RHEL/CentOS
sudo yum update kernel

# Verify patch application
dmesg | grep -i seq_file