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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org,
        syzbot+52f83f0ea8df16932f7f@syzkaller.appspotmail.com,
        "Takashi Iwai" <tiwai@suse.de>
Date:   Sat, 22 Sep 2018 01:15:42 +0100
Message-ID: <lsq.1537575342.196996512@decadent.org.uk>
Subject: [PATCH 3.16 42/63] ALSA: rawmidi: Change resized buffers atomically
In-Reply-To: <lsq.1537575341.194909669@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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

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

From: Takashi Iwai <tiwai@suse.de>

commit 39675f7a7c7e7702f7d5341f1e0d01db746543a0 upstream.

The SNDRV_RAWMIDI_IOCTL_PARAMS ioctl may resize the buffers and the
current code is racy.  For example, the sequencer client may write to
buffer while it being resized.

As a simple workaround, let's switch to the resized buffer inside the
stream runtime lock.

Reported-by: syzbot+52f83f0ea8df16932f7f@syzkaller.appspotmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 sound/core/rawmidi.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

--- a/sound/core/rawmidi.c
+++ b/sound/core/rawmidi.c
@@ -645,7 +645,7 @@ static int snd_rawmidi_info_select_user(
 int snd_rawmidi_output_params(struct snd_rawmidi_substream *substream,
 			      struct snd_rawmidi_params * params)
 {
-	char *newbuf;
+	char *newbuf, *oldbuf;
 	struct snd_rawmidi_runtime *runtime = substream->runtime;
 	
 	if (substream->append && substream->use_count > 1)
@@ -658,13 +658,17 @@ int snd_rawmidi_output_params(struct snd
 		return -EINVAL;
 	}
 	if (params->buffer_size != runtime->buffer_size) {
-		newbuf = krealloc(runtime->buffer, params->buffer_size,
-				  GFP_KERNEL);
+		newbuf = kmalloc(params->buffer_size, GFP_KERNEL);
 		if (!newbuf)
 			return -ENOMEM;
+		spin_lock_irq(&runtime->lock);
+		oldbuf = runtime->buffer;
 		runtime->buffer = newbuf;
 		runtime->buffer_size = params->buffer_size;
 		runtime->avail = runtime->buffer_size;
+		runtime->appl_ptr = runtime->hw_ptr = 0;
+		spin_unlock_irq(&runtime->lock);
+		kfree(oldbuf);
 	}
 	runtime->avail_min = params->avail_min;
 	substream->active_sensing = !params->no_active_sensing;
@@ -675,7 +679,7 @@ EXPORT_SYMBOL(snd_rawmidi_output_params)
 int snd_rawmidi_input_params(struct snd_rawmidi_substream *substream,
 			     struct snd_rawmidi_params * params)
 {
-	char *newbuf;
+	char *newbuf, *oldbuf;
 	struct snd_rawmidi_runtime *runtime = substream->runtime;
 
 	snd_rawmidi_drain_input(substream);
@@ -686,12 +690,16 @@ int snd_rawmidi_input_params(struct snd_
 		return -EINVAL;
 	}
 	if (params->buffer_size != runtime->buffer_size) {
-		newbuf = krealloc(runtime->buffer, params->buffer_size,
-				  GFP_KERNEL);
+		newbuf = kmalloc(params->buffer_size, GFP_KERNEL);
 		if (!newbuf)
 			return -ENOMEM;
+		spin_lock_irq(&runtime->lock);
+		oldbuf = runtime->buffer;
 		runtime->buffer = newbuf;
 		runtime->buffer_size = params->buffer_size;
+		runtime->appl_ptr = runtime->hw_ptr = 0;
+		spin_unlock_irq(&runtime->lock);
+		kfree(oldbuf);
 	}
 	runtime->avail_min = params->avail_min;
 	return 0;