In the Linux kernel, the following vulnerability has been resolved: ALSA: seq: midi: Serialize...

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():

snd_rawmidi_input_trigger(up=0)
				snd_midi_input_event()
				 -> snd_rawmidi_kernel_read()
				     -> snd_rawmidi_input_trigger(up=1)
  -> cancel_work_sync()

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:

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

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/[email protected]/