CVE-2021-44733: Linux Kernel Use-After-Free Vulnerability

CVE-2021-44733 Overview

CVE-2021-44733 is a use-after-free vulnerability in the TEE (Trusted Execution Environment) subsystem of the Linux kernel through version 5.15.11. The flaw exists in drivers/tee/tee_shm.c and is caused by a race condition in the tee_shm_get_from_id function during attempts to free a shared memory object. This vulnerability can be exploited by a local attacker with low privileges to potentially achieve arbitrary code execution or cause system instability.

Critical Impact

Local attackers can exploit this race condition to gain elevated privileges or execute arbitrary code by manipulating shared memory objects in the TEE subsystem, potentially compromising system integrity and confidentiality.

Affected Products

• Linux Kernel (through version 5.15.11)
• Red Hat Enterprise Linux 8.0
• Fedora 35
• Debian Linux 9.0, 10.0, and 11.0
• NetApp H410C, H300S, H500S, H700S, H300E, H500E, H700E, and H410S firmware

Discovery Timeline

• December 22, 2021 - CVE-2021-44733 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-44733

Vulnerability Analysis

This vulnerability is classified as a race condition (CWE-362) that results in a use-after-free condition. The TEE subsystem in the Linux kernel provides an interface for applications to communicate with trusted applications running in a secure environment, such as ARM TrustZone. The shared memory mechanism (tee_shm) facilitates data transfer between the normal world and the secure world.

The flaw occurs when the tee_shm_get_from_id function attempts to retrieve a shared memory object by its identifier while another thread is concurrently freeing that same object. Without proper synchronization, the function may return a reference to memory that has already been freed, leading to a use-after-free condition.

Exploitation requires local access and the ability to interact with the TEE subsystem. An attacker with low privileges who can trigger concurrent operations on shared memory objects may be able to exploit this race window to corrupt kernel memory, potentially leading to privilege escalation or arbitrary code execution in kernel context.

Root Cause

The root cause is insufficient synchronization in the tee_shm_get_from_id function when handling shared memory object lookups. The race condition exists between the reference counting mechanism and the object destruction path. When a shared memory object is being freed, there is a window where another thread can still obtain a reference to the object through tee_shm_get_from_id, resulting in access to freed memory.

Attack Vector

The attack vector is local, requiring an attacker to have low-privilege access to the system and the ability to interact with the TEE subsystem. The attacker would need to craft a scenario where multiple threads or processes perform concurrent operations on TEE shared memory objects—specifically, one thread freeing an object while another attempts to obtain a reference to it.

The vulnerability mechanism involves the following sequence:

1. A shared memory object is allocated via the TEE subsystem
2. Thread A initiates the free operation on the shared memory object
3. Before the free completes, Thread B calls tee_shm_get_from_id to retrieve the same object
4. Thread B obtains a dangling reference to the now-freed memory
5. Subsequent operations on this dangling reference can lead to memory corruption

For technical details on the vulnerability and the fix, see the Linux Kernel Commit Update.

Detection Methods for CVE-2021-44733

Indicators of Compromise

• Unexpected kernel panics or crashes related to TEE subsystem operations
• Abnormal memory access patterns in tee_shm.c module detected through kernel debugging
• Suspicious processes attempting rapid concurrent operations on TEE shared memory
• System instability when applications interact with TrustZone or OP-TEE

Detection Strategies

• Monitor kernel logs for TEE subsystem errors, memory corruption warnings, or use-after-free indicators
• Deploy kernel address sanitizer (KASAN) in test environments to detect memory violations
• Use SentinelOne Singularity Platform to monitor for behavioral indicators of privilege escalation attempts
• Implement system call auditing for TEE-related ioctl operations

Monitoring Recommendations

• Enable kernel auditing for /dev/tee* and /dev/teepriv* device access
• Monitor for unusual patterns of TEE device file operations from unprivileged users
• Track kernel module loading and TEE driver activity
• Configure alerting for kernel oops or panic events mentioning tee_shm functions

How to Mitigate CVE-2021-44733

Immediate Actions Required

• Update the Linux kernel to a patched version (5.15.12 or later, or apply backported patches)
• Apply vendor-specific security updates for your distribution (RHEL, Fedora, Debian)
• If updates cannot be immediately applied, consider restricting access to TEE device files
• Review and audit systems for signs of exploitation

Patch Information

The vulnerability was addressed in the upstream Linux kernel through commit dfd0743f1d9ea76931510ed150334d571fbab49d. This fix improves synchronization in the tee_shm_get_from_id function to prevent the race condition. Major distributions have released security updates:

• Debian Security Advisory DSA-5096
• Debian LTS Security Announcement
• NetApp Security Advisory NTAP-20220114-0003

Workarounds

• Restrict access to TEE device files (/dev/tee*, /dev/teepriv*) to only essential services and users
• Disable or unload the TEE subsystem module (optee) if not required for system operation
• Implement mandatory access control (SELinux/AppArmor) policies to limit TEE device access
• Monitor and limit concurrent access to TEE resources through application-level controls

# Restrict TEE device access to root only
chmod 600 /dev/tee*
chmod 600 /dev/teepriv*

# Optionally blacklist the optee module if not needed
echo "blacklist optee" >> /etc/modprobe.d/blacklist-tee.conf

# Unload the module if currently loaded (requires system may not use TEE)
modprobe -r optee