[PATCH v2 0/2] ALSA: hda: Widget memory fixes

[PATCH v2 0/2] ALSA: hda: Widget memory fixes

[Evan Green]

This series fixes concurrency issues with the sysfs widget array. The first
function patches up the locking that was introduced recently to protect more
of the data structure. The second patch fixes a race between a reinit and the
initial population of the array which could result in a length and array
getting out of sync.


Changes in v2:
- Introduced widget_mutex relocation

Evan Green (2):
  ALSA: hda: Fix widget_mutex incomplete protection
  ALSA: hda: Use correct start/count for sysfs init

 sound/hda/hdac_device.c | 21 ++++++++++++++-------
 sound/hda/hdac_sysfs.c  | 18 ++++++++++--------
 sound/hda/local.h       |  3 ++-
 3 files changed, 26 insertions(+), 16 deletions(-)

-- 
2.20.1

[PATCH v2 1/2] ALSA: hda: Fix widget_mutex incomplete protection

[Evan Green]

The widget_mutex was introduced to serialize callers to
hda_widget_sysfs_{re}init. However, its protection of the sysfs widget array
is incomplete. For example, it is acquired around the call to
hda_widget_sysfs_reinit(), which actually creates the new array, but isn't
still acquired when codec->num_nodes and codec->start_nid is updated. So
the lock ensures one thread sets up the new array at a time, but doesn't
ensure which thread's value will end up in codec->num_nodes. If a larger
num_nodes wins but a smaller array was set up, the next call to
refresh_widgets() will touch free memory as it iterates over codec->num_nodes
that aren't there.

The widget_lock really protects both the tree as well as codec->num_nodes,
start_nid, and end_nid, so make sure it's held across that update. It should
also be held during snd_hdac_get_sub_nodes(), so that a very old read from that
function doesn't end up clobbering a later update.

While in there, move the exit mutex call inside the function. This moves the
mutex closer to the data structure it protects and removes a requirement of
acquiring the somewhat internal widget_lock before calling sysfs_exit.

Fixes: ed180abba7f1 ("ALSA: hda: Fix race between creating and refreshing sysfs entries")

Signed-off-by: Evan Green <evgreen@chromium.org>

---

Changes in v2:
- Introduced widget_mutex relocation

 sound/hda/hdac_device.c | 19 +++++++++++++------
 sound/hda/hdac_sysfs.c  |  4 ++--
 2 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/sound/hda/hdac_device.c b/sound/hda/hdac_device.c
index 6907dbefd08c..ff3420c5cdc8 100644
--- a/sound/hda/hdac_device.c
+++ b/sound/hda/hdac_device.c
@@ -162,9 +162,7 @@ EXPORT_SYMBOL_GPL(snd_hdac_device_register);
 void snd_hdac_device_unregister(struct hdac_device *codec)
 {
 	if (device_is_registered(&codec->dev)) {
-		mutex_lock(&codec->widget_lock);
 		hda_widget_sysfs_exit(codec);
-		mutex_unlock(&codec->widget_lock);
 		device_del(&codec->dev);
 		snd_hdac_bus_remove_device(codec->bus, codec);
 	}
@@ -402,25 +400,34 @@ int snd_hdac_refresh_widgets(struct hdac_device *codec, bool sysfs)
 	hda_nid_t start_nid;
 	int nums, err;
 
+	/*
+	 * Serialize against multiple threads trying to update the sysfs
+	 * widgets array.
+	 */
+	mutex_lock(&codec->widget_lock);
 	nums = snd_hdac_get_sub_nodes(codec, codec->afg, &start_nid);
 	if (!start_nid || nums <= 0 || nums >= 0xff) {
 		dev_err(&codec->dev, "cannot read sub nodes for FG 0x%02x\n",
 			codec->afg);
-		return -EINVAL;
+		err = -EINVAL;
+		goto unlock;
 	}
 
 	if (sysfs) {
-		mutex_lock(&codec->widget_lock);
 		err = hda_widget_sysfs_reinit(codec, start_nid, nums);
-		mutex_unlock(&codec->widget_lock);
 		if (err < 0)
-			return err;
+			goto unlock;
 	}
 
 	codec->num_nodes = nums;
 	codec->start_nid = start_nid;
 	codec->end_nid = start_nid + nums;
+	mutex_unlock(&codec->widget_lock);
 	return 0;
+
+unlock:
+	mutex_unlock(&codec->widget_lock);
+	return err;
 }
 EXPORT_SYMBOL_GPL(snd_hdac_refresh_widgets);
 
diff --git a/sound/hda/hdac_sysfs.c b/sound/hda/hdac_sysfs.c
index 909d5ef1179c..400ca262e2f8 100644
--- a/sound/hda/hdac_sysfs.c
+++ b/sound/hda/hdac_sysfs.c
@@ -412,13 +412,13 @@ int hda_widget_sysfs_init(struct hdac_device *codec)
 	return 0;
 }
 
-/* call with codec->widget_lock held */
 void hda_widget_sysfs_exit(struct hdac_device *codec)
 {
+	mutex_lock(&codec->widget_lock);
 	widget_tree_free(codec);
+	mutex_unlock(&codec->widget_lock);
 }
 
-/* call with codec->widget_lock held */
 int hda_widget_sysfs_reinit(struct hdac_device *codec,
 			    hda_nid_t start_nid, int num_nodes)
 {
-- 
2.20.1

Re: [PATCH v2 1/2] ALSA: hda: Fix widget_mutex incomplete protection

[Takashi Iwai]

On Wed, 26 Jun 2019 23:22:19 +0200,
Evan Green wrote:
> 
> The widget_mutex was introduced to serialize callers to
> hda_widget_sysfs_{re}init. However, its protection of the sysfs widget array
> is incomplete. For example, it is acquired around the call to
> hda_widget_sysfs_reinit(), which actually creates the new array, but isn't
> still acquired when codec->num_nodes and codec->start_nid is updated. So
> the lock ensures one thread sets up the new array at a time, but doesn't
> ensure which thread's value will end up in codec->num_nodes. If a larger
> num_nodes wins but a smaller array was set up, the next call to
> refresh_widgets() will touch free memory as it iterates over codec->num_nodes
> that aren't there.
> 
> The widget_lock really protects both the tree as well as codec->num_nodes,
> start_nid, and end_nid, so make sure it's held across that update. It should
> also be held during snd_hdac_get_sub_nodes(), so that a very old read from that
> function doesn't end up clobbering a later update.

OK, right, this fix is needed no matter whether to take my other
change to skip hda_widget_sysfs_init() call in
hda_widget_sysfs_reinit().

However...

> While in there, move the exit mutex call inside the function. This moves the
> mutex closer to the data structure it protects and removes a requirement of
> acquiring the somewhat internal widget_lock before calling sysfs_exit.

... this doesn't look better from consistency POV.  The whole code in
hdac_sysfs.c doesn't take any lock in itself.  The protection is
supposed to be done in the caller side.  So, let's keep as is now.

Also...

>  	codec->num_nodes = nums;
>  	codec->start_nid = start_nid;
>  	codec->end_nid = start_nid + nums;
> +	mutex_unlock(&codec->widget_lock);
>  	return 0;
> +
> +unlock:
> +	mutex_unlock(&codec->widget_lock);
> +	return err;

There is no need of two mutex_unlock() here.  They can be unified,

  	codec->num_nodes = nums;
  	codec->start_nid = start_nid;
  	codec->end_nid = start_nid + nums;
  unlock:
	mutex_unlock(&codec->widget_lock);
  	return err;

Could you refresh this and resubmit?


thanks,

Takashi

Re: [PATCH v2 1/2] ALSA: hda: Fix widget_mutex incomplete protection

[Evan Green]

On Mon, Jul 1, 2019 at 7:09 AM Takashi Iwai <tiwai@suse.de> wrote:
>
> On Wed, 26 Jun 2019 23:22:19 +0200,
> Evan Green wrote:
> >
> > The widget_mutex was introduced to serialize callers to
> > hda_widget_sysfs_{re}init. However, its protection of the sysfs widget array
> > is incomplete. For example, it is acquired around the call to
> > hda_widget_sysfs_reinit(), which actually creates the new array, but isn't
> > still acquired when codec->num_nodes and codec->start_nid is updated. So
> > the lock ensures one thread sets up the new array at a time, but doesn't
> > ensure which thread's value will end up in codec->num_nodes. If a larger
> > num_nodes wins but a smaller array was set up, the next call to
> > refresh_widgets() will touch free memory as it iterates over codec->num_nodes
> > that aren't there.
> >
> > The widget_lock really protects both the tree as well as codec->num_nodes,
> > start_nid, and end_nid, so make sure it's held across that update. It should
> > also be held during snd_hdac_get_sub_nodes(), so that a very old read from that
> > function doesn't end up clobbering a later update.
>
> OK, right, this fix is needed no matter whether to take my other
> change to skip hda_widget_sysfs_init() call in
> hda_widget_sysfs_reinit().
>
> However...
>
> > While in there, move the exit mutex call inside the function. This moves the
> > mutex closer to the data structure it protects and removes a requirement of
> > acquiring the somewhat internal widget_lock before calling sysfs_exit.
>
> ... this doesn't look better from consistency POV.  The whole code in
> hdac_sysfs.c doesn't take any lock in itself.  The protection is
> supposed to be done in the caller side.  So, let's keep as is now.

Ok.

>
> Also...
>
> >       codec->num_nodes = nums;
> >       codec->start_nid = start_nid;
> >       codec->end_nid = start_nid + nums;
> > +     mutex_unlock(&codec->widget_lock);
> >       return 0;
> > +
> > +unlock:
> > +     mutex_unlock(&codec->widget_lock);
> > +     return err;
>
> There is no need of two mutex_unlock() here.  They can be unified,
>
>         codec->num_nodes = nums;
>         codec->start_nid = start_nid;
>         codec->end_nid = start_nid + nums;
>   unlock:
>         mutex_unlock(&codec->widget_lock);
>         return err;
>
> Could you refresh this and resubmit?

Sure, will do. Thanks for taking a look.

[PATCH v2 2/2] ALSA: hda: Use correct start/count for sysfs init

[Evan Green]

The normal flow through the widget sysfs codepath is that
snd_hdac_refresh_widgets() is called once without the sysfs bool set
to set up codec->num_nodes and friends, then another time with the
bool set to actually allocate all the sysfs widgets. However, during
the first time allocation, hda_widget_sysfs_reinit() ignores the new
num_nodes passed in via parameter and just calls hda_widget_sysfs_init(),
using whatever was in codec->num_nodes before the update. This is not
correct in cases where num_nodes changes. Here's an example:

Sometime earlier:
snd_hdac_refresh_widgets(hdac, false)
  sets codec->num_nodes to 2, widgets is still not allocated

Now:
snd_hdac_refresh_widgets(hdac, true)
  hda_widget_sysfs_reinit(num_nodes=7)
    hda_widget_sysfs_init()
      widget_tree_create()
        alloc(codec->num_nodes) // this is still 2
  codec->num_nodes = 7

Pass num_nodes and start_nid down into widget_tree_create() so that
the right number of nodes are allocated in all cases.

Signed-off-by: Evan Green <evgreen@chromium.org>
---

Changes in v2: None

 sound/hda/hdac_device.c |  2 +-
 sound/hda/hdac_sysfs.c  | 14 ++++++++------
 sound/hda/local.h       |  3 ++-
 3 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/sound/hda/hdac_device.c b/sound/hda/hdac_device.c
index ff3420c5cdc8..b06a698c88a1 100644
--- a/sound/hda/hdac_device.c
+++ b/sound/hda/hdac_device.c
@@ -144,7 +144,7 @@ int snd_hdac_device_register(struct hdac_device *codec)
 	if (err < 0)
 		return err;
 	mutex_lock(&codec->widget_lock);
-	err = hda_widget_sysfs_init(codec);
+	err = hda_widget_sysfs_init(codec, codec->start_nid, codec->num_nodes);
 	mutex_unlock(&codec->widget_lock);
 	if (err < 0) {
 		device_del(&codec->dev);
diff --git a/sound/hda/hdac_sysfs.c b/sound/hda/hdac_sysfs.c
index 400ca262e2f8..41aa4b98162a 100644
--- a/sound/hda/hdac_sysfs.c
+++ b/sound/hda/hdac_sysfs.c
@@ -358,7 +358,8 @@ static int add_widget_node(struct kobject *parent, hda_nid_t nid,
 	return 0;
 }
 
-static int widget_tree_create(struct hdac_device *codec)
+static int widget_tree_create(struct hdac_device *codec,
+			      hda_nid_t start_nid, int num_nodes)
 {
 	struct hdac_widget_tree *tree;
 	int i, err;
@@ -372,12 +373,12 @@ static int widget_tree_create(struct hdac_device *codec)
 	if (!tree->root)
 		return -ENOMEM;
 
-	tree->nodes = kcalloc(codec->num_nodes + 1, sizeof(*tree->nodes),
+	tree->nodes = kcalloc(num_nodes + 1, sizeof(*tree->nodes),
 			      GFP_KERNEL);
 	if (!tree->nodes)
 		return -ENOMEM;
 
-	for (i = 0, nid = codec->start_nid; i < codec->num_nodes; i++, nid++) {
+	for (i = 0, nid = start_nid; i < num_nodes; i++, nid++) {
 		err = add_widget_node(tree->root, nid, &widget_node_group,
 				      &tree->nodes[i]);
 		if (err < 0)
@@ -396,14 +397,15 @@ static int widget_tree_create(struct hdac_device *codec)
 }
 
 /* call with codec->widget_lock held */
-int hda_widget_sysfs_init(struct hdac_device *codec)
+int hda_widget_sysfs_init(struct hdac_device *codec,
+			  hda_nid_t start_nid, int num_nodes)
 {
 	int err;
 
 	if (codec->widgets)
 		return 0; /* already created */
 
-	err = widget_tree_create(codec);
+	err = widget_tree_create(codec, start_nid, num_nodes);
 	if (err < 0) {
 		widget_tree_free(codec);
 		return err;
@@ -428,7 +430,7 @@ int hda_widget_sysfs_reinit(struct hdac_device *codec,
 	int i;
 
 	if (!codec->widgets)
-		return hda_widget_sysfs_init(codec);
+		return hda_widget_sysfs_init(codec, start_nid, num_nodes);
 
 	tree = kmemdup(codec->widgets, sizeof(*tree), GFP_KERNEL);
 	if (!tree)
diff --git a/sound/hda/local.h b/sound/hda/local.h
index 877631e39373..8936120ab4d9 100644
--- a/sound/hda/local.h
+++ b/sound/hda/local.h
@@ -28,7 +28,8 @@ static inline unsigned int get_wcaps_channels(u32 wcaps)
 }
 
 extern const struct attribute_group *hdac_dev_attr_groups[];
-int hda_widget_sysfs_init(struct hdac_device *codec);
+int hda_widget_sysfs_init(struct hdac_device *codec,
+			  hda_nid_t start_nid, int num_nodes);
 int hda_widget_sysfs_reinit(struct hdac_device *codec, hda_nid_t start_nid,
 			    int num_nodes);
 void hda_widget_sysfs_exit(struct hdac_device *codec);
-- 
2.20.1