CVE-2020-27786: Linux Kernel Use-After-Free Vulnerability

CVE-2020-27786 Overview

CVE-2020-27786 is a use-after-free vulnerability [CWE-416] in the Linux kernel's Musical Instrument Digital Interface (MIDI) implementation. A local attacker with permissions to issue ioctl commands to MIDI devices can trigger the flaw. A write to freed memory before its subsequent use can redirect kernel execution flow, leading to memory corruption or privilege escalation. The vulnerability affects multiple Linux distributions and downstream products including Red Hat Enterprise Linux, OpenShift Container Platform, and NetApp appliances. The flaw resides in the snd_rawmidi subsystem and was patched in upstream commit c1f6e3c818dd734c30f6a7eeebf232ba2cf3181d.

Critical Impact
Local attackers with low privileges can corrupt kernel memory and potentially escalate to root, compromising confidentiality, integrity, and availability of affected Linux systems.

Affected Products

Linux Kernel (multiple versions prior to the patch)
Red Hat Enterprise Linux 7.0 and 8.0
Red Hat OpenShift Container Platform 4.4, 4.5, 4.6
Red Hat Enterprise MRG 2.0
NetApp Cloud Backup
NetApp SolidFire Baseboard Management Controller

Discovery Timeline

2020-12-03 - Vulnerability discussed publicly on the OpenWall oss-security list
2020-12-11 - CVE-2020-27786 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-27786

Vulnerability Analysis

The flaw originates in the Linux kernel's raw MIDI (snd_rawmidi) implementation within the ALSA sound subsystem. Local users with access to MIDI device nodes such as /dev/snd/midiC*D* can issue ioctl calls that mismanage object lifetime in kernel memory. Specifically, a kernel object is freed while still referenced, and a subsequent write into that freed region modifies memory now owned by another allocation.

This class of bug enables an attacker to overwrite kernel data structures, including function pointers, after reallocating the freed slab object with attacker-controlled content. Successful exploitation hijacks kernel execution and grants root-equivalent capabilities. The upstream fix is committed as c1f6e3c818dd734c30f6a7eeebf232ba2cf3181d in the Linus Torvalds tree.

Root Cause

The root cause is improper synchronization and reference counting in the MIDI ioctl handlers. When the kernel frees an internal MIDI substream object, lingering pointers remain reachable through concurrent ioctl paths. The absence of correct lifetime management around these objects produces the use-after-free condition described in [CWE-416].

Attack Vector

Exploitation requires a local account with permissions to open MIDI device files and issue ioctl commands. No user interaction is required. An attacker typically sprays kernel heap allocations after the free to control the contents of the reused slab, then triggers the dangling-pointer write to corrupt a target structure. See the Red Hat Bugzilla Report and the Linux Kernel Commit c1f6e3c818d for technical specifics.

No verified public proof-of-concept code is referenced in the advisory; technical details are described in the upstream commit and Red Hat tracking record.

Detection Methods for CVE-2020-27786

Indicators of Compromise

Unexpected kernel oops or panic messages referencing snd_rawmidi or snd-seq modules in dmesg or /var/log/messages.
KASAN reports flagging use-after-free in MIDI ioctl code paths on instrumented kernels.
Unprivileged processes opening /dev/snd/midiC*D* or /dev/snd/seq followed by abnormal kernel behavior or privilege transitions.

Detection Strategies

Inventory running kernel versions across endpoints and compare against vendor-fixed builds from Red Hat, upstream Linux, and NetApp advisories.
Audit which local users and containers have access to ALSA MIDI device nodes; restrict membership in the audio group on servers that do not need sound support.
Monitor for loading of the snd_rawmidi and snd_seq modules on systems that should not use audio functionality.

Monitoring Recommendations

Enable kernel audit rules to log ioctl syscalls against /dev/snd/* device files from non-interactive service accounts.
Forward kernel crash dumps and dmesg output to a centralized log platform to detect repeated use-after-free signatures.
Track sudden privilege escalations correlated with audio device access in process telemetry.

How to Mitigate CVE-2020-27786

Immediate Actions Required

Apply vendor kernel updates from Red Hat, your Linux distribution, or NetApp that include upstream commit c1f6e3c818d and reboot affected systems.
Restrict access to MIDI device nodes by removing unnecessary users from the audio group on servers and shared multi-tenant hosts.
On systems with no audio requirements, blacklist the snd_rawmidi and snd_seq kernel modules to remove the vulnerable code path.

Patch Information

The vulnerability is fixed upstream in commit c1f6e3c818dd734c30f6a7eeebf232ba2cf3181d. Red Hat tracks remediation in Red Hat Bugzilla 1900933 and NetApp publishes fixed component versions in NetApp Security Advisory ntap-20210122-0002. Customers running Red Hat Enterprise Linux 7/8, OpenShift 4.4–4.6, or Enterprise MRG 2.0 should install the corresponding errata kernel.

Workarounds

Blacklist the vulnerable modules where audio is not needed by adding blacklist snd_rawmidi and blacklist snd_seq entries under /etc/modprobe.d/.
Apply container security policies that prevent workloads from accessing /dev/snd/* device files, especially in OpenShift clusters.
Enforce least-privilege account management so that untrusted local users cannot reach MIDI ioctl interfaces.

bash

# Configuration example: disable vulnerable MIDI modules on servers without audio needs
echo 'blacklist snd_rawmidi' | sudo tee /etc/modprobe.d/disable-midi.conf
echo 'blacklist snd_seq'     | sudo tee -a /etc/modprobe.d/disable-midi.conf
sudo rmmod snd_rawmidi snd_seq 2>/dev/null
sudo update-initramfs -u  # Debian/Ubuntu, or: sudo dracut -f  (RHEL/Fedora)