linux-kernel - [PATCH 3.16 266/305] ALSA: timer: Fix negative queue usage by racy accesses

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Message-ID: <lsq.1471110171.310858209@decadent.org.uk>
Date:	Sat, 13 Aug 2016 18:42:51 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Takashi Iwai" <tiwai@...e.de>
Subject: [PATCH 3.16 266/305] ALSA: timer: Fix negative queue usage by
 racy accesses

3.16.37-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Takashi Iwai <tiwai@...e.de>

commit 3fa6993fef634e05d200d141a85df0b044572364 upstream.

The user timer tu->qused counter may go to a negative value when
multiple concurrent reads are performed since both the check and the
decrement of tu->qused are done in two individual locked contexts.
This results in bogus read outs, and the endless loop in the
user-space side.

The fix is to move the decrement of the tu->qused counter into the
same spinlock context as the zero-check of the counter.

Signed-off-by: Takashi Iwai <tiwai@...e.de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 sound/core/timer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -1965,6 +1965,7 @@ static ssize_t snd_timer_user_read(struc

 		qhead = tu->qhead++;
 		tu->qhead %= tu->queue_size;
+		tu->qused--;
 		spin_unlock_irq(&tu->qlock);

 		if (tu->tread) {
@@ -1978,7 +1979,6 @@ static ssize_t snd_timer_user_read(struc
 		}

 		spin_lock_irq(&tu->qlock);
-		tu->qused--;
 		if (err < 0)
 			goto _error;
 		result += unit;