[PATCH 4.4 050/117] ALSA: timer: Fix race at concurrent reads

Greg Kroah-Hartman
2016-02-14T22:33:29Z

From: Takashi Iwai <tiwai@suse.de>

commit 4dff5c7b7093b19c19d3a100f8a3ad87cb7cd9e7 upstream.

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.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
sound/core/timer.c | 34 +++++++++++++++-------------------
1 file changed, 15 insertions(+), 19 deletions(-)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -1920,6 +1920,7 @@ static ssize_t snd_timer_user_read(struc
{
struct snd_timer_user *tu;
long result = 0, unit;
+ int qhead;
int err = 0;

tu = file->private_data;
@@ -1931,7 +1932,7 @@ static ssize_t snd_timer_user_read(struc

if ((file->f_flags & O_NONBLOCK) != 0 || result > 0) {
err = -EAGAIN;
- break;
+ goto _error;
}

set_current_state(TASK_INTERRUPTIBLE);
@@ -1946,42 +1947,37 @@ static ssize_t snd_timer_user_read(struc

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;
}