[alsa-devel] [PATCH v3 2/2] ALSA: fireworks: accessing to user space outside spinlock

[Takashi Sakamoto]

In hwdep interface of fireworks driver, accessing to user space is in a
critical section with disabled local interrupt. Depending on architecture,
accessing to user space can cause page fault exception. Then local
processor stores machine status and handles the synchronous event. A
handler corresponding to the event can call task scheduler to wait for
preparing pages. In a case of usage of single core processor, the state to
disable local interrupt is worse because it don't handle usual interrupts
from hardware.

This commit fixes this bug, performing the accessing outside spinlock.

Reported-by: Vaishali Thakkar <vaishali.thakkar at oracle.com>
Cc: stable at vger.kernel.org
Fixes: 555e8a8f7f14('ALSA: fireworks: Add command/response functionality into hwdep interface')
Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
---
 sound/firewire/fireworks/fireworks_hwdep.c       | 52 ++++++++++++++++++------
 sound/firewire/fireworks/fireworks_transaction.c |  4 +-
 2 files changed, 42 insertions(+), 14 deletions(-)

diff --git a/sound/firewire/fireworks/fireworks_hwdep.c b/sound/firewire/fireworks/fireworks_hwdep.c
index 33df865..2563490 100644
--- a/sound/firewire/fireworks/fireworks_hwdep.c
+++ b/sound/firewire/fireworks/fireworks_hwdep.c
@@ -25,6 +25,7 @@ hwdep_read_resp_buf(struct snd_efw *efw, char __user *buf, long remained,
 {
 	unsigned int length, till_end, type;
 	struct snd_efw_transaction *t;
+	u8 *pull_ptr;
 	long count = 0;
 
 	if (remained < sizeof(type) + sizeof(struct snd_efw_transaction))
@@ -38,8 +39,17 @@ hwdep_read_resp_buf(struct snd_efw *efw, char __user *buf, long remained,
 	buf += sizeof(type);
 
 	/* write into buffer as many responses as possible */
+	spin_lock_irq(&efw->lock);
+
+	/*
+	 * When another task reaches here during this task's access to user
+	 * space, it picks up current position in buffer and can read the same
+	 * series of responses.
+	 */
+	pull_ptr = efw->pull_ptr;
+
 	while (efw->resp_queues > 0) {
-		t = (struct snd_efw_transaction *)(efw->pull_ptr);
+		t = (struct snd_efw_transaction *)(pull_ptr);
 		length = be32_to_cpu(t->length) * sizeof(__be32);
 
 		/* confirm enough space for this response */
@@ -49,16 +59,19 @@ hwdep_read_resp_buf(struct snd_efw *efw, char __user *buf, long remained,
 		/* copy from ring buffer to user buffer */
 		while (length > 0) {
 			till_end = snd_efw_resp_buf_size -
-				(unsigned int)(efw->pull_ptr - efw->resp_buf);
+				(unsigned int)(pull_ptr - efw->resp_buf);
 			till_end = min_t(unsigned int, length, till_end);
 
-			if (copy_to_user(buf, efw->pull_ptr, till_end))
+			spin_unlock_irq(&efw->lock);
+
+			if (copy_to_user(buf, pull_ptr, till_end))
 				return -EFAULT;
 
-			efw->pull_ptr += till_end;
-			if (efw->pull_ptr >= efw->resp_buf +
-					     snd_efw_resp_buf_size)
-				efw->pull_ptr -= snd_efw_resp_buf_size;
+			spin_lock_irq(&efw->lock);
+
+			pull_ptr += till_end;
+			if (pull_ptr >= efw->resp_buf + snd_efw_resp_buf_size)
+				pull_ptr -= snd_efw_resp_buf_size;
 
 			length -= till_end;
 			buf += till_end;
@@ -69,6 +82,18 @@ hwdep_read_resp_buf(struct snd_efw *efw, char __user *buf, long remained,
 		efw->resp_queues--;
 	}
 
+	/*
+	 * All of tasks can read from the buffer nearly simultaneously, but the
+	 * position of each task is different depending on the length of given
+	 * buffer. Here, for simplicity, a position of buffer is set by the
+	 * latest task. It's better for a listening application to allow one
+	 * thread to read from the buffer. Unless, each task can read different
+	 * sequence of responses depending on variation of buffer length.
+	 */
+	efw->pull_ptr = pull_ptr;
+
+	spin_unlock_irq(&efw->lock);
+
 	return count;
 }
 
@@ -76,14 +101,17 @@ static long
 hwdep_read_locked(struct snd_efw *efw, char __user *buf, long count,
 		  loff_t *offset)
 {
-	union snd_firewire_event event;
+	union snd_firewire_event event = {
+		.lock_status.type = SNDRV_FIREWIRE_EVENT_LOCK_STATUS,
+	};
 
-	memset(&event, 0, sizeof(event));
+	spin_lock_irq(&efw->lock);
 
-	event.lock_status.type = SNDRV_FIREWIRE_EVENT_LOCK_STATUS;
 	event.lock_status.status = (efw->dev_lock_count > 0);
 	efw->dev_lock_changed = false;
 
+	spin_unlock_irq(&efw->lock);
+
 	count = min_t(long, count, sizeof(event.lock_status));
 
 	if (copy_to_user(buf, &event, count))
@@ -111,13 +139,13 @@ hwdep_read(struct snd_hwdep *hwdep, char __user *buf, long count,
 		spin_lock_irq(&efw->lock);
 	}
 
+	spin_unlock_irq(&efw->lock);
+
 	if (efw->dev_lock_changed)
 		count = hwdep_read_locked(efw, buf, count, offset);
 	else if (efw->resp_queues > 0)
 		count = hwdep_read_resp_buf(efw, buf, count, offset);
 
-	spin_unlock_irq(&efw->lock);
-
 	return count;
 }
 
diff --git a/sound/firewire/fireworks/fireworks_transaction.c b/sound/firewire/fireworks/fireworks_transaction.c
index f550808..fa78e32 100644
--- a/sound/firewire/fireworks/fireworks_transaction.c
+++ b/sound/firewire/fireworks/fireworks_transaction.c
@@ -121,11 +121,11 @@ copy_resp_to_buf(struct snd_efw *efw, void *data, size_t length, int *rcode)
 	size_t capacity, till_end;
 	struct snd_efw_transaction *t;
 
-	spin_lock_irq(&efw->lock);
-
 	t = (struct snd_efw_transaction *)data;
 	length = min_t(size_t, be32_to_cpu(t->length) * sizeof(u32), length);
 
+	spin_lock_irq(&efw->lock);
+
 	if (efw->push_ptr < efw->pull_ptr)
 		capacity = (unsigned int)(efw->pull_ptr - efw->push_ptr);
 	else
-- 
2.7.4