CVE-2026-23056: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23056 Overview

A vulnerability has been identified in the Linux kernel's UACCE (Unified Accelerator Framework) subsystem. The issue stems from the uacce_vm_ops structure not implementing a custom .mremap handler, allowing the default mremap behavior to potentially cause a double free condition. When an application performs a sequence of mmap, mremap, and munmap operations, the shared vm_private_data pointer can lead to q->qfr being freed twice when both the original and remapped VMAs are unmapped.

Critical Impact

Local attackers may exploit this double free condition to cause denial of service or potentially achieve memory corruption in the Linux kernel's accelerator framework.

Affected Products

• Linux Kernel (versions with UACCE subsystem)
• Systems using hardware accelerators through the Unified Accelerator Framework
• Linux kernel builds with CONFIG_UACCE enabled

Discovery Timeline

• 2026-02-04 - CVE CVE-2026-23056 published to NVD
• 2026-02-05 - Last updated in NVD database

Technical Details for CVE-2026-23056

Vulnerability Analysis

The vulnerability exists in the UACCE (Unified Accelerator Framework) subsystem of the Linux kernel. The uacce_vm_ops structure, which defines virtual memory operations for UACCE devices, lacks an implementation of the .mremap callback function. When a driver does not implement .mremap, the kernel falls back to a default mremap method that copies the original VMA's vm_private_data pointer to the new VMA without proper handling.

This creates a dangerous scenario where the queue descriptor (q) and its associated qfr structure are referenced by multiple VMAs simultaneously. The exploitation flow involves: (1) mapping address p1 via mmap, (2) remapping to p2 via mremap, (3) unmapping p1, and (4) unmapping p2. Both munmap operations trigger vma_close, resulting in q->qfr being freed twice. While the code does set qfr to null after the first free (preventing immediate crashes in some cases), this represents a memory safety violation that could have unpredictable consequences.

Root Cause

The root cause is the absence of a .mremap handler in the uacce_vm_ops structure. The UACCE subsystem maintains per-queue memory state through vm_private_data, but the default kernel mremap behavior simply copies this pointer to new VMAs without considering the implications for the driver's resource management. The fix implements an explicit .mremap callback that returns -EPERM, preventing mremap operations entirely and informing userspace that this operation is not supported.

Attack Vector

A local attacker with access to a UACCE device (typically hardware accelerators) could trigger this vulnerability through a carefully crafted sequence of memory mapping system calls. The attack requires:

1. Opening a UACCE device file descriptor
2. Using mmap() to create an initial memory mapping
3. Calling mremap() to create a second mapping pointing to the same queue resources
4. Executing munmap() on both mappings to trigger the double free

The vulnerability is exploitable locally and requires the attacker to have permissions to access UACCE devices, which may be restricted depending on system configuration.

Detection Methods for CVE-2026-23056

Indicators of Compromise

• Kernel oops or panic messages referencing UACCE or accelerator subsystems
• Unusual patterns of mmap/mremap/munmap system calls targeting /dev/uacce/* devices
• Memory corruption signatures in kernel logs related to slab allocator double frees
• KASAN (Kernel Address Sanitizer) reports indicating use-after-free in UACCE code paths

Detection Strategies

• Enable KASAN (Kernel Address Sanitizer) in development/testing environments to detect double free conditions
• Monitor system call traces for suspicious mmap/mremap/munmap sequences on UACCE device files
• Implement audit rules for accesses to /dev/uacce/* device nodes
• Deploy kernel memory debugging options such as CONFIG_DEBUG_SLAB and CONFIG_DEBUG_PAGEALLOC

Monitoring Recommendations

• Configure kernel auditing to log all system calls to UACCE device files
• Set up alerts for kernel oops or panic events related to memory management subsystems
• Monitor for processes making unusual patterns of memory mapping operations
• Review access controls on UACCE devices to ensure only authorized users can interact with accelerator hardware

How to Mitigate CVE-2026-23056

Immediate Actions Required

• Update the Linux kernel to a patched version containing the mremap fix
• Restrict access to UACCE device files (/dev/uacce/*) to only trusted users and applications
• If UACCE functionality is not required, consider disabling the module or rebuilding the kernel without CONFIG_UACCE
• Audit applications using hardware accelerators for proper error handling of -EPERM returns from mremap

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability by implementing an explicit .mremap handler that returns -EPERM. The fix ensures that mremap operations on UACCE memory mappings are denied, preventing the double free condition. Patches are available through the official kernel git repository:

• Kernel Git Commit 0269534
• Kernel Git Commit 4c042bc
• Kernel Git Commit a407ddd
• Kernel Git Commit ba29b59

Workarounds

• Restrict access to UACCE devices by modifying udev rules to limit device permissions
• Use SELinux or AppArmor policies to prevent untrusted applications from accessing accelerator devices
• Disable the UACCE kernel module if hardware accelerator functionality is not required: modprobe -r uacce
• Apply kernel live patching solutions if available for your distribution while awaiting a full kernel update

# Restrict UACCE device access
# Add to /etc/udev/rules.d/99-uacce-restrict.rules
KERNEL=="uacce*", MODE="0600", OWNER="root", GROUP="root"

# Disable UACCE module loading
echo "blacklist uacce" >> /etc/modprobe.d/blacklist-uacce.conf