In the Linux kernel, the following vulnerability has been resolved:
ALSA: seq: midi: Serialize input teardown with event_input
snd_midi_input_event() must not be running while a rawmidi substream is
closing, since this can lead to the trigger state becoming out-of-step
through this sequence in snd_rawmidi_input_trigger():
<pre>
snd_rawmidi_input_trigger(up=0)
snd_midi_input_event()
-> snd_rawmidi_kernel_read()
-> snd_rawmidi_input_trigger(up=1)
-> cancel_work_sync()
</pre>
which ends with the underlying device being active unexpectedly.
When this is called from close_substream(), further input can re-trigger
the input event leaving it running after rawmidi_release_priv() has set
rfile->rmidi to NULL which leads to:
<pre>
Unable to handle kernel NULL pointer dereference at virtual address 00000000000000b0
Call trace:
snd_midi_input_event+0x3c/0x134 [snd_seq_midi] (P)
snd_rawmidi_input_event_work+0x1c/0x2c
process_one_work+0x150/0x3a4
worker_thread+0x190/0x318
</pre>
Apply a similar approach to commit ef7607ab1c8ad ("ALSA: seq: midi:
Serialize output teardown with event_input") which fixed the same issue
in the output direction, but updated to use RCU following Takashi Iwai's
proposed follow-on patch [1].
With this change in place, midisynth_unsubscribe() clears the input file
so snd_midi_input_event() will not re-trigger the stream and will be
quiesced by the cancel_work_sync() in snd_rawmidi_input_trigger().
[1] https://lore.kernel.org/linux-sound/20260813144224.753399-1-tiwai@suse.de/