GHSA-WP68-7GHH-V4RV
Vulnerability from github – Published: 2026-08-15 06:32 – Updated: 2026-08-17 06:33In 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
{
"affected": [],
"aliases": [
"CVE-2026-74387"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-08-15T06:22:40Z",
"severity": "HIGH"
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nALSA: seq: midi: Serialize output teardown with event_input\n\nevent_process_midi() borrows msynth-\u003eoutput_rfile.output and then\npasses the substream to dump_midi() and snd_rawmidi_kernel_write()\nwithout synchronizing with the output open/close transition.\nmidisynth_use() also publishes output_rfile before\nsnd_rawmidi_output_params() has finished.\n\nThe last midisynth_unuse() can therefore release the same rawmidi file\nand free substream-\u003eruntime before snd_rawmidi_kernel_write1() takes\nits runtime buffer reference. That leaves the event_input path using a\nstale substream or runtime and can end in a NULL-deref or use-after-free.\n\nFix this with two pieces of synchronization. Keep a short IRQ-safe\nspinlock only for publishing or clearing output_rfile and for pairing\nthe output snapshot with an snd_use_lock_t reference. Once\nevent_process_midi() has taken that in-flight reference, it drops the\nspinlock before calling snd_seq_dump_var_event(), dump_midi(), or\nsnd_rawmidi_kernel_write(). midisynth_unuse() now detaches the visible\nrawmidi file under the same spinlock, waits for the in-flight writers\nto drain, and only then drains and releases the saved file.\nmidisynth_use() likewise opens into a local snd_rawmidi_file and\npublishes it only after snd_rawmidi_output_params() succeeds.\n\nThe buggy scenario involves two paths, with each column showing the\norder within that path:\n\nevent_input path: last unuse path:\n1. event_process_midi() snapshots 1. midisynth_unuse() starts\n output_rfile.output. tearing down output_rfile.\n2. dump_midi() reaches 2. snd_rawmidi_kernel_release()\n snd_rawmidi_kernel_write() closes the output file.\n before runtime is pinned. 3. close_substream() frees\n3. The callback keeps using substream-\u003eruntime.\n the borrowed substream.\n\nValidation reproduced this kernel report:\nKASAN null-ptr-deref in snd_rawmidi_kernel_write1+0x56/0x360\nRIP: 0033:0x7fde7dd0837f\nRIP: 0010:snd_rawmidi_kernel_write1+0x56/0x360",
"id": "GHSA-wp68-7ghh-v4rv",
"modified": "2026-08-17T06:33:42Z",
"published": "2026-08-15T06:32:32Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-74387"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/11165fe2c5ea0516debe486d91df67abbe36905e"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/718f6a56b40875f19e6915799044a02df9abfd52"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/936641af564c3d92721704b781e36aaf223efdd2"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/d6fd2afb137f52bf00c5210cc44d08ed54dcffb4"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/ef7607ab1c8adc6258fb1b27d08e26aecdc18a58"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/f5d470b808bc01f70978e22e595c6f7768313406"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
"type": "CVSS_V3"
}
]
}
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