[PATCH 1/2] ALSA: emux: Fix UAF of soundfont zones at note-on

[Takashi Iwai <tiwai@suse.de>]

snd_emux_note_on() looks up the matching zones via
snd_soundfont_search_zone(), which takes sflist->lock only during the
lookup and returns the raw zone pointers after releasing it.  The
caller then stores them in vp->zone without any protection.
Meanwhile, a concurrent SNDRV_EMUX_IOCTL_REMOVE_LAST_SAMPLES ioctl may
unlink and free those zones in snd_soundfont_remove_unlocked(); this
leads to a use-after-free in snd_emux_note_on(), and later at
accessing the cached vp->zone, e.g. in snd_emux_send_effect().

Fix it by holding sflist->lock in snd_emux_note_on() over the whole
zone lookup and voice setup, and let snd_soundfont_search_zone()
require the caller to hold the lock, instead.
snd_soundfont_lock_preset() takes sflist->lock for setting
presets_locked, so the removal path now syncs with the note-on finish.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/bug/linux-697212bb-a434-4ff5-8778-25a84f5dad89
Cc: stable@vger.kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 sound/synth/emux/emux_synth.c | 3 +++
 sound/synth/emux/soundfont.c  | 9 ++++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/sound/synth/emux/emux_synth.c b/sound/synth/emux/emux_synth.c
index 6982e1317da5..2cbf12364a1e 100644
--- a/sound/synth/emux/emux_synth.c
+++ b/sound/synth/emux/emux_synth.c
@@ -60,6 +60,9 @@ snd_emux_note_on(void *p, int note, int vel, struct snd_midi_channel *chan)
 		return;
 
 	key = note; /* remember the original note */
+
+	/* hold sflist->lock for protecting against concurrent free */
+	guard(spinlock_irqsave)(&emu->sflist->lock);
 	nvoices = get_zone(emu, port, &note, vel, chan, table);
 	if (! nvoices)
 		return;
diff --git a/sound/synth/emux/soundfont.c b/sound/synth/emux/soundfont.c
index c172b53b17a4..039357d96e07 100644
--- a/sound/synth/emux/soundfont.c
+++ b/sound/synth/emux/soundfont.c
@@ -1225,6 +1225,9 @@ delete_preset(struct snd_sf_list *sflist, struct snd_sf_zone *zp)
  * The found zones are stored on 'table' array.  max_layers defines
  * the maximum number of elements in this array.
  * This function returns the number of found zones.  0 if not found.
+ *
+ * The caller must hold sflist->lock, and keep holding it as long as
+ * the returned zones are accessed.
  */
 int
 snd_soundfont_search_zone(struct snd_sf_list *sflist, int *notep, int vel,
@@ -1234,11 +1237,11 @@ snd_soundfont_search_zone(struct snd_sf_list *sflist, int *notep, int vel,
 {
 	int nvoices;
 
-	/* this function is supposed to be called atomically,
-	 * so we check the lock.  if it's busy, just returns 0 to
+	lockdep_assert_held(&sflist->lock);
+
+	/* if the presets are being modified, just returns 0 to
 	 * tell the caller the busy state
 	 */
-	guard(spinlock_irqsave)(&sflist->lock);
 	if (sflist->presets_locked)
 		return 0;
 	nvoices = search_zones(sflist, notep, vel, preset, bank,
-- 
2.55.0