CVE-2023-1077: Linux Kernel Use-After-Free Vulnerability

CVE-2023-1077 Overview

A type confusion vulnerability exists in the Linux kernel's real-time (RT) scheduler subsystem. The pick_next_rt_entity() function may return a type confused entry that is not properly detected by the BUG_ON condition. This occurs because the confused entry is not NULL but rather a list_head structure, which then gets incorrectly used as a sched_rt_entity, leading to memory corruption.

Critical Impact

Local attackers with low privileges can exploit this vulnerability to cause memory corruption, potentially leading to privilege escalation or system compromise on affected Linux systems.

Affected Products

• Linux Kernel (multiple versions)
• Debian Linux 10.0
• NetApp A700s, 8300, 8700, A400, C400 series firmware
• NetApp H300s, H500s, H700s, H410s, H410c series firmware

Discovery Timeline

• March 27, 2023 - CVE-2023-1077 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-1077

Vulnerability Analysis

This vulnerability represents a type confusion flaw (CWE-843) within the Linux kernel's real-time scheduling mechanism. The root issue lies in the pick_next_rt_entity() function, which is responsible for selecting the next schedulable entity in the RT scheduling class. Under certain conditions, this function can return a pointer that represents a list_head structure rather than the expected sched_rt_entity structure.

The kernel's existing safety check using BUG_ON fails to detect this condition because it only validates whether the returned pointer is NULL. Since the type confused entry is a valid non-NULL pointer (pointing to a list_head), the check passes, and the kernel proceeds to treat this memory region as a sched_rt_entity structure.

This type confusion allows subsequent operations on what the kernel believes is a scheduling entity to actually manipulate unrelated kernel memory structures, potentially enabling an attacker to corrupt critical kernel data structures.

Root Cause

The vulnerability stems from insufficient type validation in the RT scheduler's entity selection logic. The BUG_ON condition was designed to catch NULL pointer returns but did not account for the possibility of a type confused entry being returned. The list_head structure shares some characteristics with valid scheduler entities (non-NULL pointer), allowing it to bypass the safety check while being fundamentally incompatible with the expected data layout.

Attack Vector

Exploitation requires local access to the system with at least low-level user privileges. An attacker would need to manipulate the scheduling state to trigger the condition where pick_next_rt_entity() returns a type confused entry. Due to the high attack complexity required to reliably trigger this race condition in the scheduler, exploitation is non-trivial but achievable under certain workload conditions.

The vulnerability affects the confidentiality, integrity, and availability of the system, as memory corruption in kernel scheduler structures can lead to arbitrary read/write primitives, privilege escalation, or system crashes.

Detection Methods for CVE-2023-1077

Indicators of Compromise

• Unexpected kernel panics or crashes related to the RT scheduler subsystem
• Anomalous behavior in processes using real-time scheduling policies (SCHED_FIFO, SCHED_RR)
• Kernel log messages indicating memory corruption or invalid scheduler entity access
• Unexpected changes to process privileges or security contexts

Detection Strategies

• Monitor kernel logs for scheduler-related oops, panics, or BUG() assertions
• Implement runtime integrity monitoring for kernel memory structures
• Deploy kernel-level endpoint detection that can identify anomalous scheduler behavior
• Use SentinelOne's behavioral AI to detect exploitation attempts targeting kernel vulnerabilities

Monitoring Recommendations

• Enable kernel auditing for real-time scheduling system calls (sched_setscheduler, sched_setattr)
• Monitor for unusual patterns of RT scheduling operations from non-privileged processes
• Deploy crash dump analysis to identify scheduler-related memory corruption events
• Implement continuous kernel integrity verification on critical systems

How to Mitigate CVE-2023-1077

Immediate Actions Required

• Apply the latest kernel security updates from your distribution vendor
• Review systems using real-time scheduling for potential exploitation indicators
• Consider restricting RT scheduling capabilities to trusted processes using ulimit or cgroups
• Prioritize patching for systems running latency-sensitive or real-time workloads

Patch Information

The vulnerability has been addressed in the upstream Linux kernel. The fix can be found in the Linux Kernel Commit. Distribution-specific patches are available through:

• Debian LTS Advisory (May 2023)
• Debian LTS Advisory (January 2024)
• NetApp Security Advisory NTAP-20230511-0002

Workarounds

• Restrict real-time scheduling privileges using sysctl kernel.sched_rt_runtime_us settings
• Limit RT scheduling capabilities to specific users/groups via /etc/security/limits.conf
• Use container isolation (cgroups v2) to restrict RT scheduling access for untrusted workloads
• Consider disabling RT scheduler features if not required for your workload

# Restrict RT scheduling capabilities
# Add to /etc/security/limits.conf
* hard rtprio 0
* soft rtprio 0

# Or limit RT bandwidth via sysctl
sysctl -w kernel.sched_rt_runtime_us=950000
sysctl -w kernel.sched_rt_period_us=1000000