CVE-2026-31588: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31588 Overview

CVE-2026-31588 is a use-after-free vulnerability in the Linux kernel's KVM (Kernel-based Virtual Machine) x86 MMIO emulation subsystem. The flaw occurs when the emulator initiates a write operation using an on-stack local variable as the source, and the write crosses a page boundary where both pages are MMIO pages. Due to KVM's ABI constraints that only allow physically contiguous MMIO requests, accesses that split MMIO pages are separated into two fragments and sent to userspace one at a time. When KVM attempts to complete the second fragment after the initial KVM_RUN returns, it references the now-invalid stack memory, resulting in a use-after-free condition.

Critical Impact
This vulnerability enables local attackers with KVM access to potentially achieve code execution with elevated privileges by exploiting the use-after-free condition in MMIO fragment handling, particularly when separate tasks handle sequential KVM_RUN operations.

Affected Products

Linux Kernel (multiple versions with KVM x86 support)
Systems running KVM-based virtualization (QEMU/KVM)
Linux distributions using affected kernel versions

Discovery Timeline

April 24, 2026 - CVE-2026-31588 published to NVD
April 28, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31588

Vulnerability Analysis

The vulnerability exists in the KVM x86 MMIO emulation code, specifically in the complete_emulated_mmio() function. When KVM needs to emulate an MMIO write operation, it may use on-stack local variables to hold the data to be written. The problem arises when such a write operation crosses a page boundary and both destination pages are MMIO-mapped.

KVM's design limitation requires that MMIO requests be physically contiguous, forcing page-boundary-crossing accesses to be split into two separate fragments. Each fragment is processed in a separate userspace exit/re-entry cycle via KVM_RUN ioctl. After the first fragment completes and userspace re-enters with KVM_RUN, KVM detects the pending second fragment and attempts to service it—but the original on-stack source buffer may no longer be valid.

The KASAN (Kernel Address Sanitizer) report in the CVE description confirms the memory corruption:

BUG: KASAN: use-after-free in complete_emulated_mmio+0x305/0x420
Read of size 1 at addr ffff888009c378d1 by task syz-executor417/984

This demonstrates that freed stack memory is being accessed during the MMIO completion phase.

Root Cause

The root cause is improper memory lifecycle management in the MMIO fragment handling code. When the emulator stores a pointer directly to an on-stack variable for the write data source, that pointer becomes a dangling reference after the stack frame is unwound. The vulnerability is exacerbated when the second KVM_RUN is performed by a different task entirely, as the initiating task's stack may contain completely unrelated data or be marked as freed memory.

The fix introduces a scratch field in the MMIO fragment structure that copies the write value (for payloads 8 bytes or less) rather than storing a pointer to the source, ensuring the data remains valid across userspace exits.

Attack Vector

Exploitation requires local access with the ability to interact with KVM virtual machines. An attacker would need to:

Create or access a VM with MMIO-mapped memory regions
Craft memory access patterns that trigger page-boundary-crossing MMIO writes
Manipulate the timing or task context of KVM_RUN operations to trigger the use-after-free
Potentially leverage the memory corruption for code execution or information disclosure

The vulnerability has a local attack vector and requires low privileges (KVM access), but can potentially escape the VM boundary to affect the host system.

Detection Methods for CVE-2026-31588

Indicators of Compromise

KASAN warnings in kernel logs containing use-after-free in complete_emulated_mmio
Unexpected VM crashes or host system instability during MMIO-heavy workloads
Anomalous memory access patterns in KVM-related kernel code paths
Kernel oops or panics referencing kvm_arch_vcpu_ioctl_run in the call stack

Detection Strategies

Enable KASAN (Kernel Address Sanitizer) on development and test systems to detect memory corruption
Monitor kernel logs for KVM-related memory errors or KASAN reports
Deploy kernel runtime integrity monitoring to detect exploitation attempts
Implement system call auditing for KVM_RUN ioctl operations on sensitive systems

Monitoring Recommendations

Configure syslog monitoring for kernel memory corruption indicators
Implement alerting on unusual KVM error patterns or VM stability issues
Monitor for processes making rapid sequential KVM_RUN calls with MMIO activity
Use eBPF-based monitoring to track KVM subsystem behavior anomalies

How to Mitigate CVE-2026-31588

Immediate Actions Required

Update the Linux kernel to a patched version that includes the MMIO scratch field fix
Review and restrict access to KVM interfaces to trusted users only
Monitor systems running KVM workloads for signs of exploitation attempts
Consider temporarily disabling KVM on highly sensitive systems until patching is complete

Patch Information

Multiple patches have been released across different kernel stable branches. The fix copies the to-be-written value to a scratch field in the MMIO fragment when the payload is 8 bytes or less, preventing dangling pointer references. Relevant patches are available at:

Workarounds

Restrict KVM module loading to essential systems only using kernel module blacklisting
Implement strict access controls on /dev/kvm device to limit exposure
Use SELinux or AppArmor policies to confine processes with KVM access
Consider network isolation for virtualization hosts until patches are applied

bash

# Restrict KVM device access to specific group
chmod 660 /dev/kvm
chown root:kvm /dev/kvm

# Add only trusted users to kvm group
usermod -aG kvm trusted_user

# Optional: Blacklist KVM module on non-virtualization systems
echo "blacklist kvm" >> /etc/modprobe.d/blacklist-kvm.conf
echo "blacklist kvm_intel" >> /etc/modprobe.d/blacklist-kvm.conf
echo "blacklist kvm_amd" >> /etc/modprobe.d/blacklist-kvm.conf