linux-kernel - [PATCH 3.16 091/366] ALSA: pcm: Fix UAF at PCM release via PCM timer access

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Message-ID: <lsq.1539530741.643402943@decadent.org.uk>
Date:   Sun, 14 Oct 2018 16:25:41 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        syzbot+8e62ff4e07aa2ce87826@...kaller.appspotmail.com,
        "Takashi Iwai" <tiwai@...e.de>
Subject: [PATCH 3.16 091/366] ALSA: pcm: Fix UAF at PCM release via PCM
 timer access

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

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

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

commit a820ccbe21e8ce8e86c39cd1d3bc8c7d1cbb949b upstream.

The PCM runtime object is created and freed dynamically at PCM stream
open / close time.  This is tracked via substream->runtime, and it's
cleared at snd_pcm_detach_substream().

The runtime object assignment is protected by PCM open_mutex, so for
all PCM operations, it's safely handled.  However, each PCM substream
provides also an ALSA timer interface, and user-space can access to
this while closing a PCM substream.  This may eventually lead to a
UAF, as snd_pcm_timer_resolution() tries to access the runtime while
clearing it in other side.

Fortunately, it's the only concurrent access from the PCM timer, and
it merely reads runtime->timer_resolution field.  So, we can avoid the
race by reordering kfree() and wrapping the substream->runtime
clearance with the corresponding timer lock.

Reported-by: syzbot+8e62ff4e07aa2ce87826@...kaller.appspotmail.com
Signed-off-by: Takashi Iwai <tiwai@...e.de>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 sound/core/pcm.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -28,6 +28,7 @@
 #include <sound/core.h>
 #include <sound/minors.h>
 #include <sound/pcm.h>
+#include <sound/timer.h>
 #include <sound/control.h>
 #include <sound/info.h>

@@ -1002,8 +1003,13 @@ void snd_pcm_detach_substream(struct snd
 #ifdef CONFIG_SND_PCM_XRUN_DEBUG
 	kfree(runtime->hwptr_log);
 #endif
-	kfree(runtime);
+	/* Avoid concurrent access to runtime via PCM timer interface */
+	if (substream->timer)
+		spin_lock_irq(&substream->timer->lock);
 	substream->runtime = NULL;
+	if (substream->timer)
+		spin_unlock_irq(&substream->timer->lock);
+	kfree(runtime);
 	put_pid(substream->pid);
 	substream->pid = NULL;
 	substream->pstr->substream_opened--;