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ALSA: seq: midi: Serialize input teardown with event_input

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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 ef7607ab1c ("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/

Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Link: https://patch.msgid.link/20260813150810.795393-1-jkeeping@inmusicbrands.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
This commit is contained in:
John Keeping 2026-08-13 16:08:08 +01:00 committed by Takashi Iwai
parent 9895573b01
commit 403f7f3ad3
1 changed files with 31 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

37
sound/core/seq/seq_midi.c
View File

@ -42,6 +42,8 @@ struct seq_midisynth {
struct snd_rawmidi *rmidi;
int device;
int subdevice;
struct snd_rawmidi_substream __rcu *input_substream;
snd_use_lock_t input_use_lock; /* in-flight event_input users */
struct snd_rawmidi_file input_rfile;
snd_use_lock_t output_use_lock; /* in-flight event_input users */
struct snd_rawmidi_substream __rcu *output_substream;
@ -76,6 +78,14 @@ static void snd_midi_input_event(struct snd_rawmidi_substream *substream)
msynth = runtime->private_data;
if (msynth == NULL)
return;
scoped_guard(rcu) {
if (rcu_dereference(msynth->input_substream) != substream)
return;
snd_use_lock_use(&msynth->input_use_lock);
}
memset(&ev, 0, sizeof(ev));
while (runtime->avail > 0) {
res = snd_rawmidi_kernel_read(substream, buf, sizeof(buf));
@ -95,6 +105,8 @@ static void snd_midi_input_event(struct snd_rawmidi_substream *substream)
memset(&ev, 0, sizeof(ev));
}
}
snd_use_lock_free(&msynth->input_use_lock);
}
static int dump_midi(struct snd_rawmidi_substream *substream, const char *buf, int count)
@ -177,6 +189,7 @@ static int snd_seq_midisynth_new(struct seq_midisynth *msynth,
msynth->card = card;
msynth->device = device;
msynth->subdevice = subdevice;
snd_use_lock_init(&msynth->input_use_lock);
snd_use_lock_init(&msynth->output_use_lock);
return 0;
}
@ -187,28 +200,31 @@ static int midisynth_subscribe(void *private_data, struct snd_seq_port_subscribe
int err;
struct seq_midisynth *msynth = private_data;
struct snd_rawmidi_runtime *runtime;
struct snd_rawmidi_file rfile = {};
struct snd_rawmidi_params params;
/* open midi port */
err = snd_rawmidi_kernel_open(msynth->rmidi, msynth->subdevice,
SNDRV_RAWMIDI_LFLG_INPUT,
&msynth->input_rfile);
&rfile);
if (err < 0) {
pr_debug("ALSA: seq_midi: midi input open failed!!!\n");
return err;
}
runtime = msynth->input_rfile.input->runtime;
runtime = rfile.input->runtime;
memset(&params, 0, sizeof(params));
params.avail_min = 1;
params.buffer_size = input_buffer_size;
err = snd_rawmidi_input_params(msynth->input_rfile.input, &params);
err = snd_rawmidi_input_params(rfile.input, &params);
if (err < 0) {
snd_rawmidi_kernel_release(&msynth->input_rfile);
snd_rawmidi_kernel_release(&rfile);
return err;
}
snd_midi_event_reset_encode(msynth->parser);
runtime->event = snd_midi_input_event;
runtime->private_data = msynth;
msynth->input_rfile = rfile;
rcu_assign_pointer(msynth->input_substream, rfile.input);
snd_rawmidi_kernel_read(msynth->input_rfile.input, NULL, 0);
return 0;
}
@ -218,10 +234,19 @@ static int midisynth_unsubscribe(void *private_data, struct snd_seq_port_subscri
{
int err;
struct seq_midisynth *msynth = private_data;
struct snd_rawmidi_file rfile;
if (snd_BUG_ON(!msynth->input_rfile.input))
rcu_assign_pointer(msynth->input_substream, NULL);
synchronize_rcu();
snd_use_lock_sync(&msynth->input_use_lock);
rfile = msynth->input_rfile;
msynth->input_rfile = (struct snd_rawmidi_file){};
if (snd_BUG_ON(!rfile.input))
return -EINVAL;
err = snd_rawmidi_kernel_release(&msynth->input_rfile);
err = snd_rawmidi_kernel_release(&rfile);
return err;
}