[PATCH 3/3] drm/exynos: vidi: use ctx->lock to protect struct vidi_context member variables related to memory alloc/free

[Jeongjun Park <aha310510@xxxxxxxxx>]

Exynos Virtual Display driver performs memory allocation/free operations
without lock protection, which easily causes concurrency problem.

For example, use-after-free can occur in race scenario like this:
```
	CPU0				CPU1				CPU2
	----				----				----
  vidi_connection_ioctl()
    if (vidi->connection) // true
      drm_edid = drm_edid_alloc(); // alloc drm_edid
      ...
      ctx->raw_edid = drm_edid;
      ...
								drm_mode_getconnector()
								  drm_helper_probe_single_connector_modes()
								    vidi_get_modes()
								      if (ctx->raw_edid) // true
								        drm_edid_dup(ctx->raw_edid);
								          if (!drm_edid) // false
								          ...
				vidi_connection_ioctl()
				  if (vidi->connection) // false
				    drm_edid_free(ctx->raw_edid); // free drm_edid
				    ...
								          drm_edid_alloc(drm_edid->edid)
								            kmemdup(edid); // UAF!!
								            ...
```

To prevent these vulns, at least in vidi_context, member variables related
to memory alloc/free should be protected with ctx->lock.

Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
---
 drivers/gpu/drm/exynos/exynos_drm_vidi.c | 38 ++++++++++++++++++++----
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/exynos/exynos_drm_vidi.c b/drivers/gpu/drm/exynos/exynos_drm_vidi.c
index 601406b640c7..37733f2ac0e7 100644
--- a/drivers/gpu/drm/exynos/exynos_drm_vidi.c
+++ b/drivers/gpu/drm/exynos/exynos_drm_vidi.c
@@ -186,29 +186,37 @@ static ssize_t vidi_store_connection(struct device *dev,
 				const char *buf, size_t len)
 {
 	struct vidi_context *ctx = dev_get_drvdata(dev);
-	int ret;
+	int ret, new_connected;
 
-	ret = kstrtoint(buf, 0, &ctx->connected);
+	ret = kstrtoint(buf, 0, &new_connected);
 	if (ret)
 		return ret;
-
-	if (ctx->connected > 1)
+	if (new_connected > 1)
 		return -EINVAL;
 
+	mutex_lock(&ctx->lock);
+
 	/*
 	 * Use fake edid data for test. If raw_edid is set then it can't be
 	 * tested.
 	 */
 	if (ctx->raw_edid) {
 		DRM_DEV_DEBUG_KMS(dev, "edid data is not fake data.\n");
-		return -EINVAL;
+		ret = -EINVAL;
+		goto fail;
 	}
 
+	ctx->connected = new_connected;
+	mutex_unlock(&ctx->lock);
+
 	DRM_DEV_DEBUG_KMS(dev, "requested connection.\n");
 
 	drm_helper_hpd_irq_event(ctx->drm_dev);
 
 	return len;
+fail:
+	mutex_unlock(&ctx->lock);
+	return ret;
 }
 
 static DEVICE_ATTR(connection, 0644, vidi_show_connection,
@@ -243,11 +251,14 @@ int vidi_connection_ioctl(struct drm_device *drm_dev, void *data,
 		return -EINVAL;
 	}
 
+	mutex_lock(&ctx->lock);
 	if (ctx->connected == vidi->connection) {
+		mutex_unlock(&ctx->lock);
 		DRM_DEV_DEBUG_KMS(ctx->dev,
 				  "same connection request.\n");
 		return -EINVAL;
 	}
+	mutex_unlock(&ctx->lock);
 
 	if (vidi->connection) {
 		const struct drm_edid *drm_edid;
@@ -281,14 +292,21 @@ int vidi_connection_ioctl(struct drm_device *drm_dev, void *data,
 					  "edid data is invalid.\n");
 			return -EINVAL;
 		}
+		mutex_lock(&ctx->lock);
 		ctx->raw_edid = drm_edid;
+		mutex_unlock(&ctx->lock);
 	} else {
 		/* with connection = 0, free raw_edid */
+		mutex_lock(&ctx->lock);
 		drm_edid_free(ctx->raw_edid);
 		ctx->raw_edid = NULL;
+		mutex_unlock(&ctx->lock);
 	}
 
+	mutex_lock(&ctx->lock);
 	ctx->connected = vidi->connection;
+	mutex_unlock(&ctx->lock);
+
 	drm_helper_hpd_irq_event(ctx->drm_dev);
 
 	return 0;
@@ -303,7 +321,7 @@ static enum drm_connector_status vidi_detect(struct drm_connector *connector,
 	 * connection request would come from user side
 	 * to do hotplug through specific ioctl.
 	 */
-	return ctx->connected ? connector_status_connected :
+	return READ_ONCE(ctx->connected) ? connector_status_connected :
 			connector_status_disconnected;
 }
 
@@ -326,11 +344,15 @@ static int vidi_get_modes(struct drm_connector *connector)
 	const struct drm_edid *drm_edid;
 	int count;
 
+	mutex_lock(&ctx->lock);
+
 	if (ctx->raw_edid)
 		drm_edid = drm_edid_dup(ctx->raw_edid);
 	else
 		drm_edid = drm_edid_alloc(fake_edid_info, sizeof(fake_edid_info));
 
+	mutex_unlock(&ctx->lock);
+
 	drm_edid_connector_update(connector, drm_edid);
 
 	count = drm_edid_connector_add_modes(connector);
@@ -482,9 +504,13 @@ static void vidi_remove(struct platform_device *pdev)
 {
 	struct vidi_context *ctx = platform_get_drvdata(pdev);
 
+	mutex_lock(&ctx->lock);
+
 	drm_edid_free(ctx->raw_edid);
 	ctx->raw_edid = NULL;
 
+	mutex_unlock(&ctx->lock);
+
 	component_del(&pdev->dev, &vidi_component_ops);
 }
 
--