[PATCH v2] ALSA: firewire-motu: fix buffer overflow in hwdep read for DSP events

[Junrui Luo]

The DSP event handling code in hwdep_read() could write more bytes to
the user buffer than requested, when a user provides a buffer smaller
than the event header size (8 bytes).

In the put_user() loop that copies event data, when the user buffer
size is not aligned to 4 bytes, it could overwrite beyond the buffer
boundary.

Fix by:
- using min_t() to clamp the copy size
- Adding a bounds check before put_user()

This ensures we never write more than the user requested.

Fixes: 634ec0b2906e ("ALSA: firewire-motu: notify event for parameter change in register DSP model")
Signed-off-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
---
Changes in v2:
- Also fix the similar issue in the put_user() loop suggested by Takashi Iwai
- Link to v1: https://lore.kernel.org/all/SYBPR01MB78810656377E79E58350D951AFD9A@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#u
---
 sound/firewire/motu/motu-hwdep.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/sound/firewire/motu/motu-hwdep.c b/sound/firewire/motu/motu-hwdep.c
index 981c19430cb0..89dc436a0652 100644
--- a/sound/firewire/motu/motu-hwdep.c
+++ b/sound/firewire/motu/motu-hwdep.c
@@ -75,7 +75,7 @@ static long hwdep_read(struct snd_hwdep *hwdep, char __user *buf, long count,
 		while (consumed < count &&
 		       snd_motu_register_dsp_message_parser_copy_event(motu, &ev)) {
 			ptr = (u32 __user *)(buf + consumed);
-			if (put_user(ev, ptr))
+			if (consumed + sizeof(ev) > count || put_user(ev, ptr))
 				return -EFAULT;
 			consumed += sizeof(ev);
 		}
@@ -83,10 +83,11 @@ static long hwdep_read(struct snd_hwdep *hwdep, char __user *buf, long count,
 		event.motu_register_dsp_change.type = SNDRV_FIREWIRE_EVENT_MOTU_REGISTER_DSP_CHANGE;
 		event.motu_register_dsp_change.count =
 			(consumed - sizeof(event.motu_register_dsp_change)) / 4;
-		if (copy_to_user(buf, &event, sizeof(event.motu_register_dsp_change)))
+		if (copy_to_user(buf, &event,
+				 min_t(long, count, sizeof(event.motu_register_dsp_change))))
 			return -EFAULT;
 
-		count = consumed;
+		count = min_t(long, count, consumed);
 	} else {
 		spin_unlock_irq(&motu->lock);
 

---
base-commit: 559e608c46553c107dbba19dae0854af7b219400
change-id: 20251208-fixes-2d11c1218a5f

Best regards,
-- 
Junrui Luo <moonafterrain@xxxxxxxxxxx>