[PATCH 3/6] ALSA: timer: fix lockless tu->tread read in snd_timer_user_read()

[Omer Cohen <nevergfx1@gmail.com>]

snd_timer_user_read() reads tu->tread without holding ioctl_lock to
determine the entry size (unit), then reads it again under ioctl_lock
to select the copy format.  A concurrent SNDRV_TIMER_IOCTL_TREAD64
can change tu->tread between the two reads, causing a mismatch between
the entry size used for the read loop and the format used for the
actual copy.

Move the initial tu->tread read inside the ioctl_lock and cache the
value in a local variable for consistent use throughout the function.

Fixes: d11662f4f798 ("ALSA: timer: Fix race between read and ioctl")
Cc: stable@vger.kernel.org
Reported-by: Omer Cohen <nevergfx1@gmail.com>
Signed-off-by: Omer Cohen <nevergfx1@gmail.com>
---
 sound/core/timer.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/sound/core/timer.c b/sound/core/timer.c
index 51c6ac4df9f4..XXXXXXXXXXXX 100644
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -2390,12 +2390,15 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
 	struct snd_timer_tread32 tread32;
 	struct snd_timer_user *tu;
 	long result = 0, unit;
+	int tread_format;
 	int qhead;
 	int err = 0;

 	tu = file->private_data;
-	switch (tu->tread) {
-	case TREAD_FORMAT_TIME64:
+	mutex_lock(&tu->ioctl_lock);
+	tread_format = tu->tread;
+	switch (tread_format) {
+	case TREAD_FORMAT_TIME64:
 		unit = sizeof(struct snd_timer_tread64);
 		break;
 	case TREAD_FORMAT_TIME32:
@@ -2407,8 +2410,8 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
 	default:
 		WARN_ONCE(1, "Corrupt snd_timer_user\n");
+		mutex_unlock(&tu->ioctl_lock);
 		return -ENOTSUPP;
 	}

-	mutex_lock(&tu->ioctl_lock);
 	spin_lock_irq(&tu->qlock);
 	while ((long)count - result >= unit) {
@@ -2453,7 +2456,7 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,

 		tread = &tu->tqueue[qhead];

-		switch (tu->tread) {
+		switch (tread_format) {
 		case TREAD_FORMAT_TIME64:
 			if (copy_to_user(buffer, tread,
 					 sizeof(struct snd_timer_tread64)))
--
2.43.0