[PATCH] ALSA: firewire-motu: fix invalid memory access when operating hwdep character device

[Takashi Sakamoto]

ALSA firewire-motu driver recently got support for event notification via
ALSA HwDep interface for register DSP models. However, when polling ALSA
HwDep cdev, the driver can cause null pointer dereference for the other
models due to accessing to unallocated memory or uninitialized memory.

This commit fixes the bug by check the type of model before accessing to
the memory.

Fixes: 634ec0b2906e ("ALSA: firewire-motu: notify event for parameter change in register DSP model")
Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
---
 sound/firewire/motu/motu-hwdep.c | 72 ++++++++++++++++++++------------
 1 file changed, 45 insertions(+), 27 deletions(-)

diff --git a/sound/firewire/motu/motu-hwdep.c b/sound/firewire/motu/motu-hwdep.c
index 9c2e457ce692..ae2d01ddc8d3 100644
--- a/sound/firewire/motu/motu-hwdep.c
+++ b/sound/firewire/motu/motu-hwdep.c
@@ -16,6 +16,47 @@
 
 #include "motu.h"
 
+static bool has_dsp_event(struct snd_motu *motu)
+{
+	if (motu->spec->flags & SND_MOTU_SPEC_REGISTER_DSP)
+		return (snd_motu_register_dsp_message_parser_count_event(motu) > 0);
+	else
+		return false;
+}
+
+// NOTE: Take care of page fault due to accessing to userspace.
+static long copy_dsp_event_to_user(struct snd_motu *motu, char __user *buf, long count,
+				   struct snd_firewire_event_motu_register_dsp_change *event)
+{
+	if (motu->spec->flags & SND_MOTU_SPEC_REGISTER_DSP) {
+		size_t consumed = 0;
+		u32 __user *ptr;
+		u32 ev;
+
+		// Header is filled later.
+		consumed += sizeof(*event);
+
+		while (consumed < count &&
+		       snd_motu_register_dsp_message_parser_copy_event(motu, &ev)) {
+			ptr = (u32 __user *)(buf + consumed);
+			if (put_user(ev, ptr))
+				return -EFAULT;
+			consumed += sizeof(ev);
+		}
+
+		event->type = SNDRV_FIREWIRE_EVENT_MOTU_REGISTER_DSP_CHANGE;
+		event->count = (consumed - sizeof(*event)) / 4;
+		if (copy_to_user(buf, &event, sizeof(*event)))
+			return -EFAULT;
+
+		count = consumed;
+	} else {
+		count = 0;
+	}
+
+	return count;
+}
+
 static long hwdep_read(struct snd_hwdep *hwdep, char __user *buf, long count,
 		       loff_t *offset)
 {
@@ -25,8 +66,7 @@ static long hwdep_read(struct snd_hwdep *hwdep, char __user *buf, long count,
 
 	spin_lock_irq(&motu->lock);
 
-	while (!motu->dev_lock_changed && motu->msg == 0 &&
-			snd_motu_register_dsp_message_parser_count_event(motu) == 0) {
+	while (!motu->dev_lock_changed && motu->msg == 0 && !has_dsp_event(motu)) {
 		prepare_to_wait(&motu->hwdep_wait, &wait, TASK_INTERRUPTIBLE);
 		spin_unlock_irq(&motu->lock);
 		schedule();
@@ -55,31 +95,10 @@ static long hwdep_read(struct snd_hwdep *hwdep, char __user *buf, long count,
 		count = min_t(long, count, sizeof(event));
 		if (copy_to_user(buf, &event, count))
 			return -EFAULT;
-	} else if (snd_motu_register_dsp_message_parser_count_event(motu) > 0) {
-		size_t consumed = 0;
-		u32 __user *ptr;
-		u32 ev;
-
+	} else if (has_dsp_event(motu)) {
 		spin_unlock_irq(&motu->lock);
 
-		// Header is filled later.
-		consumed += sizeof(event.motu_register_dsp_change);
-
-		while (consumed < count &&
-		       snd_motu_register_dsp_message_parser_copy_event(motu, &ev)) {
-			ptr = (u32 __user *)(buf + consumed);
-			if (put_user(ev, ptr))
-				return -EFAULT;
-			consumed += sizeof(ev);
-		}
-
-		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)))
-			return -EFAULT;
-
-		count = consumed;
+		count = copy_dsp_event_to_user(motu, buf, count, &event.motu_register_dsp_change);
 	}
 
 	return count;
@@ -94,8 +113,7 @@ static __poll_t hwdep_poll(struct snd_hwdep *hwdep, struct file *file,
 	poll_wait(file, &motu->hwdep_wait, wait);
 
 	spin_lock_irq(&motu->lock);
-	if (motu->dev_lock_changed || motu->msg ||
-	    snd_motu_register_dsp_message_parser_count_event(motu) > 0)
+	if (motu->dev_lock_changed || motu->msg || has_dsp_event(motu))
 		events = EPOLLIN | EPOLLRDNORM;
 	else
 		events = 0;
-- 
2.30.2