CVE-2023-46862: Linux Kernel Race Condition Vulnerability

CVE-2023-46862 Overview

CVE-2023-46862 is a Null Pointer Dereference vulnerability discovered in the Linux kernel through version 6.5.9. The flaw exists in the io_uring/fdinfo.c file where a race condition between SQ (Submission Queue) thread exit and the io_uring_show_fdinfo function can lead to a NULL pointer dereference, potentially causing a denial of service condition on affected systems.

Critical Impact

Local attackers with low privileges can exploit this race condition to cause system instability or denial of service by triggering a NULL pointer dereference during io_uring fdinfo operations.

Affected Products

• Linux Kernel versions through 6.5.9
• Systems utilizing io_uring subsystem functionality
• Debian-based distributions (see Debian LTS Announcement)

Discovery Timeline

• October 29, 2023 - CVE-2023-46862 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-46862

Vulnerability Analysis

This vulnerability stems from a race condition in the Linux kernel's io_uring subsystem. The io_uring_show_fdinfo function in io_uring/fdinfo.c can encounter a NULL pointer when accessing SQ thread data while the SQ thread is concurrently exiting. This timing-sensitive bug requires precise race condition exploitation but can be triggered by local users with limited privileges.

The io_uring subsystem is a high-performance asynchronous I/O interface in the Linux kernel. When reading fdinfo (file descriptor information) for an io_uring instance, the kernel attempts to retrieve CPU and PID information from the associated SQ thread. Without proper synchronization, this operation can race against the SQ thread's exit path, resulting in access to freed or NULL memory.

Root Cause

The root cause is inadequate synchronization when accessing SQ thread data structures during fdinfo retrieval. The original code did not properly lock the SQ thread while retrieving thread CPU/PID information, creating a window where the SQ thread could exit and deallocate its data structures before the fdinfo operation completed.

Attack Vector

The attack vector is local, requiring an attacker to have low-privilege access to the system. The attacker must be able to:

1. Create an io_uring instance with an associated SQ thread
2. Trigger concurrent access to the fdinfo interface while manipulating the SQ thread lifecycle
3. Win the race condition to cause the NULL pointer dereference

The high attack complexity reflects the difficulty of reliably winning this race condition.

 __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *f)
 {
 	struct io_ring_ctx *ctx = f->private_data;
-	struct io_sq_data *sq = NULL;
 	struct io_overflow_cqe *ocqe;
 	struct io_rings *r = ctx->rings;
 	unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;

Source: GitHub Linux Commit Record

Detection Methods for CVE-2023-46862

Indicators of Compromise

• Kernel panic or oops messages referencing io_uring_show_fdinfo in the call stack
• System crashes or unexpected reboots in environments heavily utilizing io_uring
• Kernel logs showing NULL pointer dereference in io_uring/fdinfo.c

Detection Strategies

• Monitor kernel logs for NULL pointer dereference exceptions related to io_uring components
• Implement kernel crash dump analysis to identify exploitation attempts
• Track processes accessing /proc/[pid]/fdinfo/ for io_uring file descriptors with unusual patterns
• Deploy kernel-level monitoring for race condition exploitation signatures

Monitoring Recommendations

• Enable kernel crash reporting and analysis tools such as kdump
• Monitor system stability metrics for unexpected kernel crashes
• Review audit logs for suspicious io_uring usage patterns
• Configure alerting for kernel oops events involving io_uring subsystem

How to Mitigate CVE-2023-46862

Immediate Actions Required

• Update the Linux kernel to a patched version that includes commit 7644b1a1c9a7ae8ab99175989bfc8676055edb46
• Apply security patches from your distribution vendor (e.g., Debian LTS updates)
• Consider temporarily disabling io_uring functionality if patching is not immediately possible
• Restrict local user access on systems running vulnerable kernels

Patch Information

The vulnerability has been addressed in the Linux kernel through commit 7644b1a1c9a7ae8ab99175989bfc8676055edb46. The fix adds proper locking of the SQ thread while retrieving thread CPU/PID information in the io_uring_show_fdinfo function. This ensures that the SQ thread cannot exit while its data is being accessed.

Relevant patch resources:

• GitHub Linux Commit Record
• Kernel Bugzilla ID #218032
• Debian LTS Announcement

Workarounds

• Disable io_uring system-wide by setting io_uring_disabled sysctl parameter on supported kernel versions
• Restrict io_uring access using seccomp filters to limit which processes can use the subsystem
• Limit local user access to reduce the attack surface for local privilege exploitation

# Disable io_uring system-wide (requires kernel support)
echo 2 > /proc/sys/kernel/io_uring_disabled

# Verify io_uring is disabled
cat /proc/sys/kernel/io_uring_disabled