ALSA: usb-audio: Fix parse of UAC2 Extension Units

Extension Unit (XU) is used to have a compatible layout with
Processing Unit (PU) on UAC1, and the usb-audio driver code assumed it
for parsing the descriptors.  Meanwhile, on UAC2, XU became slightly
incompatible with PU; namely, XU has a one-byte bmControls bitmap
while PU has two bytes bmControls bitmap.  This incompatibility
results in the read of a wrong address for the last iExtension field,
which ended up with an incorrect string for the mixer element name, as
recently reported for Focusrite Scarlett 18i20 device.

This patch corrects this misalignment by introducing a couple of new
macros and calling them depending on the descriptor type.

Fixes: 23caaf19b11e ("ALSA: usb-mixer: Add support for Audio Class v2.0")
Reported-by: Stefan Sauer <ensonic@hora-obscura.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 10 insertions, 6 deletions

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index e003b5e7b01a..ac121b10c51c 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -2318,7 +2318,7 @@ static struct procunit_info extunits[] = {
  */
 static int build_audio_procunit(struct mixer_build *state, int unitid,
 				void *raw_desc, struct procunit_info *list,
-				char *name)
+				bool extension_unit)
 {
 	struct uac_processing_unit_descriptor *desc = raw_desc;
 	int num_ins;
@@ -2335,6 +2335,8 @@ static int build_audio_procunit(struct mixer_build *state, int unitid,
 	static struct procunit_info default_info = {
 		0, NULL, default_value_info
 	};
+	const char *name = extension_unit ?
+		"Extension Unit" : "Processing Unit";
 
 	if (desc->bLength < 13) {
 		usb_audio_err(state->chip, "invalid %s descriptor (id %d)\n", name, unitid);
@@ -2448,7 +2450,10 @@ static int build_audio_procunit(struct mixer_build *state, int unitid,
 		} else if (info->name) {
 			strlcpy(kctl->id.name, info->name, sizeof(kctl->id.name));
 		} else {
-			nameid = uac_processing_unit_iProcessing(desc, state->mixer->protocol);
+			if (extension_unit)
+				nameid = uac_extension_unit_iExtension(desc, state->mixer->protocol);
+			else
+				nameid = uac_processing_unit_iProcessing(desc, state->mixer->protocol);
 			len = 0;
 			if (nameid)
 				len = snd_usb_copy_string_desc(state->chip,
@@ -2481,10 +2486,10 @@ static int parse_audio_processing_unit(struct mixer_build *state, int unitid,
 	case UAC_VERSION_2:
 	default:
 		return build_audio_procunit(state, unitid, raw_desc,
-				procunits, "Processing Unit");
+					    procunits, false);
 	case UAC_VERSION_3:
 		return build_audio_procunit(state, unitid, raw_desc,
-				uac3_procunits, "Processing Unit");
+					    uac3_procunits, false);
 	}
 }
 
@@ -2495,8 +2500,7 @@ static int parse_audio_extension_unit(struct mixer_build *state, int unitid,
 	 * Note that we parse extension units with processing unit descriptors.
 	 * That's ok as the layout is the same.
 	 */
-	return build_audio_procunit(state, unitid, raw_desc,
-				    extunits, "Extension Unit");
+	return build_audio_procunit(state, unitid, raw_desc, extunits, true);
 }
 
 /*