[PATCH 1/4] ALSA: caiaq: Serialize access to the EP1 command buffer

[Niko Huuskonen]

snd_usb_caiaq_send_command() and snd_usb_caiaq_send_command_bank() copy
the command into cdev->ep1_out_buf and send it with a synchronous bulk
transfer. Nothing serializes their callers. An ALSA control write, which
sets the LEDs on the Kore controllers and several other devices, can run
at the same time as a PCM prepare, which sends the audio parameters
through the same buffer. One caller can then overwrite the buffer while
the transfer of the other is still in flight, and the device receives a
mix of both commands.

Protect the buffer with a mutex. All callers run in process context and
already sleep in usb_bulk_msg().

The problem was found by code review while adding another user of the
buffer, the Kore LCD support later in this series. It has not been
observed or reproduced.

Fixes: 8e3cd08ed8e5 ("[ALSA] caiaq - add control API and more input features")
Assisted-by: LLM
Signed-off-by: Niko Huuskonen <niko.huuskonen.00@xxxxxxxxx>
---
 sound/usb/caiaq/device.c | 5 +++++
 sound/usb/caiaq/device.h | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/sound/usb/caiaq/device.c b/sound/usb/caiaq/device.c
index a16e59248480..3e63eecebe00 100644
--- a/sound/usb/caiaq/device.c
+++ b/sound/usb/caiaq/device.c
@@ -212,6 +212,8 @@ int snd_usb_caiaq_send_command(struct snd_usb_caiaqdev *cdev,
 	if (len > EP1_BUFSIZE - 1)
 		len = EP1_BUFSIZE - 1;
 
+	guard(mutex)(&cdev->ep1_out_mutex);
+
 	if (buffer && len > 0)
 		memcpy(cdev->ep1_out_buf+1, buffer, len);
 
@@ -235,6 +237,8 @@ int snd_usb_caiaq_send_command_bank(struct snd_usb_caiaqdev *cdev,
 	if (len > EP1_BUFSIZE - 2)
 		len = EP1_BUFSIZE - 2;
 
+	guard(mutex)(&cdev->ep1_out_mutex);
+
 	if (buffer && len > 0)
 		memcpy(cdev->ep1_out_buf+2, buffer, len);
 
@@ -439,6 +443,7 @@ static int create_card(struct usb_device *usb_dev,
 	cdev->chip.usb_id = USB_ID(le16_to_cpu(usb_dev->descriptor.idVendor),
 				  le16_to_cpu(usb_dev->descriptor.idProduct));
 	spin_lock_init(&cdev->spinlock);
+	mutex_init(&cdev->ep1_out_mutex);
 
 	*cardp = card;
 	return 0;
diff --git a/sound/usb/caiaq/device.h b/sound/usb/caiaq/device.h
index 743eb0387b5f..0354e348e919 100644
--- a/sound/usb/caiaq/device.h
+++ b/sound/usb/caiaq/device.h
@@ -2,6 +2,8 @@
 #ifndef CAIAQ_DEVICE_H
 #define CAIAQ_DEVICE_H
 
+#include <linux/mutex.h>
+
 #include "../usbaudio.h"
 
 #define USB_VID_NATIVEINSTRUMENTS 0x17cc
@@ -68,6 +70,7 @@ struct snd_usb_caiaqdev {
 
 	unsigned char ep1_in_buf[EP1_BUFSIZE];
 	unsigned char ep1_out_buf[EP1_BUFSIZE];
+	struct mutex ep1_out_mutex;	/* protects ep1_out_buf */
 	unsigned char midi_out_buf[EP1_BUFSIZE];
 
 	struct caiaq_device_spec spec;
-- 
2.55.0