CVE-2026-23432: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23432 Overview

A use-after-free vulnerability has been identified in the Linux kernel's mshv (Microsoft Hypervisor) subsystem. The flaw exists in the error path of the mshv_map_user_memory() function, where improper memory management leads to a dangerous memory safety condition. When the function encounters an error, it calls vfree() directly on the memory region without first unregistering the MMU (Memory Management Unit) notifier. This leaves a dangling reference that can be triggered when userspace later unmaps the associated memory.

Critical Impact

When userspace unmaps memory after the error condition, the still-registered MMU notifier fires and attempts to access the already-freed memory region, resulting in a use-after-free condition that can cause kernel panic and potential privilege escalation.

Affected Products

• Linux kernel with mshv (Microsoft Hypervisor) support enabled
• Systems running Linux kernel as a guest under Microsoft Hypervisor

Discovery Timeline

• 2026-04-03 - CVE-2026-23432 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-23432

Vulnerability Analysis

This use-after-free vulnerability occurs within the mshv_map_user_memory() function in the Linux kernel's Microsoft Hypervisor (mshv) driver. The function is responsible for mapping user memory regions for use with the hypervisor interface. During normal operation, when a memory region is mapped, an MMU notifier is registered to track changes to the memory mapping.

The vulnerability manifests specifically in the error handling path of this function. When an error occurs during the memory mapping process, the function incorrectly calls vfree() directly to free the allocated memory region. However, this approach fails to unregister the MMU notifier that was associated with the region during setup.

The consequence is that the MMU notifier remains registered in the kernel's notification chain, holding a stale pointer to the freed memory region. Subsequently, when userspace attempts to unmap the original memory (a normal operation), the kernel's MMU notification system triggers the registered notifier callback. This callback then dereferences the stale pointer to access the freed region, resulting in a classic use-after-free condition.

Root Cause

The root cause is improper resource cleanup in the error path of mshv_map_user_memory(). The function uses vfree() for memory deallocation when it should use mshv_partition_put(), which properly handles the complete teardown sequence including MMU notifier unregistration. This oversight creates a resource lifecycle mismatch where the memory is freed but associated kernel resources (the MMU notifier registration) remain active.

Attack Vector

The vulnerability requires local access to a system running the Linux kernel with mshv support. An attacker with the ability to interact with the mshv interface could potentially:

1. Trigger the error path in mshv_map_user_memory() by providing specially crafted parameters or by manipulating system state
2. Wait for or trigger the userspace memory unmap operation
3. Exploit the resulting use-after-free to corrupt kernel memory or achieve code execution in kernel context

The vulnerability could lead to denial of service through kernel panic or potentially privilege escalation if the freed memory is reallocated with attacker-controlled content before the dangling notifier fires.

Detection Methods for CVE-2026-23432

Indicators of Compromise

• Kernel panic messages referencing mshv subsystem or MMU notifier callbacks
• Kernel oops or crashes during virtual machine memory operations
• Unexpected system reboots on systems running under Microsoft Hypervisor
• KASAN (Kernel Address Sanitizer) reports indicating use-after-free in mshv-related code paths

Detection Strategies

• Enable KASAN in kernel builds to detect use-after-free conditions at runtime
• Monitor kernel logs for oops or panics originating from mshv driver code
• Deploy kernel debugging tools such as SLUB debugging or KFENCE to catch memory corruption
• Implement system call auditing for mshv-related ioctls to detect exploitation attempts

Monitoring Recommendations

• Configure kernel crash dump collection to capture evidence of exploitation attempts
• Set up alerting for kernel panic events, particularly those mentioning mshv or MMU notifiers
• Monitor system stability metrics for virtualized workloads running under Microsoft Hypervisor
• Review system logs for unusual mshv driver activity or repeated error conditions

How to Mitigate CVE-2026-23432

Immediate Actions Required

• Update to a patched Linux kernel version that includes the fix
• Review and audit any systems running Linux as a guest under Microsoft Hypervisor
• Consider restricting access to mshv interfaces if updates cannot be immediately applied
• Enable kernel memory debugging features to detect potential exploitation attempts

Patch Information

The fix replaces the incorrect vfree() call with mshv_partition_put() in the error path, ensuring proper cleanup of all associated resources including the MMU notifier. Patches are available in the kernel git repository:

• Kernel Git Commit 34861bdc0c01
• Kernel Git Commit 6922db250422

Workarounds

• Disable mshv kernel module if Microsoft Hypervisor support is not required
• Restrict access to mshv device files using filesystem permissions
• Run workloads in containers or sandboxed environments with limited kernel interface access
• Implement mandatory access control policies (SELinux/AppArmor) to restrict mshv ioctl access

# Configuration example
# Disable mshv module loading
echo "blacklist mshv" >> /etc/modprobe.d/blacklist-mshv.conf

# Remove loaded module if safe to do so
rmmod mshv 2>/dev/null || true

# Restrict device permissions if module must remain loaded
chmod 600 /dev/mshv