[Patch V2] v4l2-core: fix use-after-free error

[Sumit Gupta <sumitg@nvidia.com>]

From: sumitg <sumitg@nvidia.com>

Fixing use-after-free within __v4l2_ctrl_handler_setup().
Memory is being freed with kfree(new_ref) for duplicate
control reference entry but cluster is still referring
to the duplicate entry. Change done to update ctrl->cluster
only when new control reference is added. Also, checking if
ctrl->ncontrols is non-zero before accessing controls from
cluster pointer to avoid illegal access when cluster has
zero controls.

 ==================================================================
 BUG: KASAN: use-after-free in __v4l2_ctrl_handler_setup+0x388/0x428
 Read of size 8 at addr ffffffc324e78618 by task systemd-udevd/312

 Allocated by task 312:

 Freed by task 312:

 The buggy address belongs to the object at ffffffc324e78600
  which belongs to the cache kmalloc-64 of size 64
 The buggy address is located 24 bytes inside of
  64-byte region [ffffffc324e78600, ffffffc324e78640)
 The buggy address belongs to the page:
 page:ffffffbf0c939e00 count:1 mapcount:0 mapping:
					(null) index:0xffffffc324e78f80
 flags: 0x4000000000000100(slab)
 raw: 4000000000000100 0000000000000000 ffffffc324e78f80 000000018020001a
 raw: 0000000000000000 0000000100000001 ffffffc37040fb80 0000000000000000
 page dumped because: kasan: bad access detected

 Memory state around the buggy address:
  ffffffc324e78500: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
  ffffffc324e78580: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
 >ffffffc324e78600: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
                             ^
  ffffffc324e78680: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
  ffffffc324e78700: 00 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc
 ==================================================================

Suggested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
---
v2:
* update ctrl->cluster only when new control reference is added.
* check ctrl->ncontrols to avoid illegal access when cluster has zero controls.

 drivers/media/v4l2-core/v4l2-ctrls.c | 53 +++++++++++++++++++-----------------
 1 file changed, 28 insertions(+), 25 deletions(-)

diff --git a/drivers/media/v4l2-core/v4l2-ctrls.c b/drivers/media/v4l2-core/v4l2-ctrls.c
index 5e3806f..3d0a952 100644
--- a/drivers/media/v4l2-core/v4l2-ctrls.c
+++ b/drivers/media/v4l2-core/v4l2-ctrls.c
@@ -2154,15 +2154,6 @@ static int handler_new_ref(struct v4l2_ctrl_handler *hdl,
 	if (size_extra_req)
 		new_ref->p_req.p = &new_ref[1];
 
-	if (ctrl->handler == hdl) {
-		/* By default each control starts in a cluster of its own.
-		   new_ref->ctrl is basically a cluster array with one
-		   element, so that's perfect to use as the cluster pointer.
-		   But only do this for the handler that owns the control. */
-		ctrl->cluster = &new_ref->ctrl;
-		ctrl->ncontrols = 1;
-	}
-
 	INIT_LIST_HEAD(&new_ref->node);
 
 	mutex_lock(hdl->lock);
@@ -2195,6 +2186,15 @@ static int handler_new_ref(struct v4l2_ctrl_handler *hdl,
 	hdl->buckets[bucket] = new_ref;
 	if (ctrl_ref)
 		*ctrl_ref = new_ref;
+	if (ctrl->handler == hdl) {
+		/* By default each control starts in a cluster of its own.
+		 * new_ref->ctrl is basically a cluster array with one
+		 * element, so that's perfect to use as the cluster pointer.
+		 * But only do this for the handler that owns the control.
+		 */
+		ctrl->cluster = &new_ref->ctrl;
+		ctrl->ncontrols = 1;
+	}
 
 unlock:
 	mutex_unlock(hdl->lock);
@@ -2725,25 +2725,28 @@ int __v4l2_ctrl_handler_setup(struct v4l2_ctrl_handler *hdl)
 		ctrl->done = false;
 
 	list_for_each_entry(ctrl, &hdl->ctrls, node) {
-		struct v4l2_ctrl *master = ctrl->cluster[0];
-		int i;
-
-		/* Skip if this control was already handled by a cluster. */
-		/* Skip button controls and read-only controls. */
-		if (ctrl->done || ctrl->type == V4L2_CTRL_TYPE_BUTTON ||
-		    (ctrl->flags & V4L2_CTRL_FLAG_READ_ONLY))
-			continue;
+		if (ctrl->ncontrols) {
+			struct v4l2_ctrl *master = ctrl->cluster[0];
+			int i;
+
+			/* Skip if this control was already handled by a */
+			/* cluster. Skip button controls and read-only */
+			/* controls. */
+			if (ctrl->done || ctrl->type == V4L2_CTRL_TYPE_BUTTON ||
+				(ctrl->flags & V4L2_CTRL_FLAG_READ_ONLY))
+				continue;
 
-		for (i = 0; i < master->ncontrols; i++) {
-			if (master->cluster[i]) {
-				cur_to_new(master->cluster[i]);
-				master->cluster[i]->is_new = 1;
-				master->cluster[i]->done = true;
+			for (i = 0; i < master->ncontrols; i++) {
+				if (master->cluster[i]) {
+					cur_to_new(master->cluster[i]);
+					master->cluster[i]->is_new = 1;
+					master->cluster[i]->done = true;
+				}
 			}
+			ret = call_op(master, s_ctrl);
+			if (ret)
+				break;
 		}
-		ret = call_op(master, s_ctrl);
-		if (ret)
-			break;
 	}
 
 	return ret;
-- 
2.7.4