ALSA: timer: Fix race at concurrent reads

snd_timer_user_read() has a potential race among parallel reads, as
qhead and qused are updated outside the critical section due to
copy_to_user() calls.  Move them into the critical section, and also
sanitize the relevant code a bit.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/timer.c b/sound/core/timer.c
index a0405b0078c62a48fc039a9ae628f658fa1d4311..dca817fc78941b5f9109e8117b3fc5adb621562e 100644 (file)
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -1933,6 +1933,7 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
 {
        struct snd_timer_user *tu;
        long result = 0, unit;
+       int qhead;
        int err = 0;
 
        tu = file->private_data;
@@ -1944,7 +1945,7 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
 
                        if ((file->f_flags & O_NONBLOCK) != 0 || result > 0) {
                                err = -EAGAIN;
-                               break;
+                               goto _error;
                        }
 
                        set_current_state(TASK_INTERRUPTIBLE);
@@ -1959,42 +1960,37 @@ static ssize_t snd_timer_user_read(struct file *file, char __user *buffer,
 
                        if (tu->disconnected) {
                                err = -ENODEV;
-                               break;
+                               goto _error;
                        }
                        if (signal_pending(current)) {
                                err = -ERESTARTSYS;
-                               break;
+                               goto _error;
                        }
                }
 
+               qhead = tu->qhead++;
+               tu->qhead %= tu->queue_size;
                spin_unlock_irq(&tu->qlock);
-               if (err < 0)
-                       goto _error;
 
                if (tu->tread) {
-                       if (copy_to_user(buffer, &tu->tqueue[tu->qhead++],
-                                        sizeof(struct snd_timer_tread))) {
+                       if (copy_to_user(buffer, &tu->tqueue[qhead],
+                                        sizeof(struct snd_timer_tread)))
                                err = -EFAULT;
-                               goto _error;
-                       }
                } else {
-                       if (copy_to_user(buffer, &tu->queue[tu->qhead++],
-                                        sizeof(struct snd_timer_read))) {
+                       if (copy_to_user(buffer, &tu->queue[qhead],
+                                        sizeof(struct snd_timer_read)))
                                err = -EFAULT;
-                               goto _error;
-                       }
                }
 
-               tu->qhead %= tu->queue_size;
-
-               result += unit;
-               buffer += unit;
-
                spin_lock_irq(&tu->qlock);
                tu->qused--;
+               if (err < 0)
+                       goto _error;
+               result += unit;
+               buffer += unit;
        }
-       spin_unlock_irq(&tu->qlock);
  _error:
+       spin_unlock_irq(&tu->qlock);
        return result > 0 ? result : err;
 }