drm/cma-helper: Fix crash in fbdev error path

Sergey Suloev reported a crash happening in drm_client_dev_hotplug()
when fbdev had failed to register.

[    9.124598] vc4_hdmi 3f902000.hdmi: ASoC: Failed to create component debugfs directory
[    9.147667] vc4_hdmi 3f902000.hdmi: vc4-hdmi-hifi <-> 3f902000.hdmi mapping ok
[    9.155184] vc4_hdmi 3f902000.hdmi: ASoC: no DMI vendor name!
[    9.166544] vc4-drm soc:gpu: bound 3f902000.hdmi (ops vc4_hdmi_ops [vc4])
[    9.173840] vc4-drm soc:gpu: bound 3f806000.vec (ops vc4_vec_ops [vc4])
[    9.181029] vc4-drm soc:gpu: bound 3f004000.txp (ops vc4_txp_ops [vc4])
[    9.188519] vc4-drm soc:gpu: bound 3f400000.hvs (ops vc4_hvs_ops [vc4])
[    9.195690] vc4-drm soc:gpu: bound 3f206000.pixelvalve (ops vc4_crtc_ops [vc4])
[    9.203523] vc4-drm soc:gpu: bound 3f207000.pixelvalve (ops vc4_crtc_ops [vc4])
[    9.215032] vc4-drm soc:gpu: bound 3f807000.pixelvalve (ops vc4_crtc_ops [vc4])
[    9.274785] vc4-drm soc:gpu: bound 3fc00000.v3d (ops vc4_v3d_ops [vc4])
[    9.290246] [drm] Initialized vc4 0.0.0 20140616 for soc:gpu on minor 0
[    9.297464] [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
[    9.304600] [drm] Driver supports precise vblank timestamp query.
[    9.382856] vc4-drm soc:gpu: [drm:drm_fb_helper_fbdev_setup [drm_kms_helper]] *ERROR* Failed to set fbdev configuration
[   10.404937] Unable to handle kernel paging request at virtual address 00330a656369768a
[   10.441620] [00330a656369768a] address between user and kernel address ranges
[   10.449087] Internal error: Oops: 96000004 [#1] PREEMPT SMP
[   10.454762] Modules linked in: brcmfmac vc4 drm_kms_helper cfg80211 drm rfkill smsc95xx brcmutil usbnet drm_panel_orientation_quirks raspberrypi_hwmon bcm2835_dma crc32_ce pwm_bcm2835 bcm2835_rng virt_dma rng_core i2c_bcm2835 ip_tables x_tables ipv6
[   10.477296] CPU: 2 PID: 45 Comm: kworker/2:1 Not tainted 4.19.0-rc5 #3
[   10.483934] Hardware name: Raspberry Pi 3 Model B Rev 1.2 (DT)
[   10.489966] Workqueue: events output_poll_execute [drm_kms_helper]
[   10.596515] Process kworker/2:1 (pid: 45, stack limit = 0x000000007e8924dc)
[   10.603590] Call trace:
[   10.606259]  drm_client_dev_hotplug+0x5c/0xb0 [drm]
[   10.611303]  drm_kms_helper_hotplug_event+0x30/0x40 [drm_kms_helper]
[   10.617849]  output_poll_execute+0xc4/0x1e0 [drm_kms_helper]
[   10.623616]  process_one_work+0x1c8/0x318
[   10.627695]  worker_thread+0x48/0x428
[   10.631420]  kthread+0xf8/0x128
[   10.634615]  ret_from_fork+0x10/0x18
[   10.638255] Code: 54000220 f9401261 aa1303e0 b4000141 (f9400c21)
[   10.644456] ---[ end trace c75b4a4b0e141908 ]---

The reason for this is that drm_fbdev_cma_init() removes the drm_client
when fbdev registration fails, but it doesn't remove the client from the
drm_device client list. So the client list now has a pointer that points
into the unknown and we have a 'use after free' situation.

Split drm_client_new() into drm_client_init() and drm_client_add() to fix
removal in the error path.

Fixes: 894a677f ("drm/cma-helper: Use the generic fbdev emulation")
Reported-by: Sergey Suloev <ssuloev@orpaltech.com>
Cc: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181001194536.57756-1-noralf@tronnes.org

drivers/gpu/drm/drm_client.c

@@ -63,20 +63,21 @@ static void drm_client_close(struct drm_client_dev *client)
 EXPORT_SYMBOL(drm_client_close);
 
 /**
- * drm_client_new - Create a DRM client
+ * drm_client_init - Initialise a DRM client
  * @dev: DRM device
  * @client: DRM client
  * @name: Client name
  * @funcs: DRM client functions (optional)
  *
+ * This initialises the client and opens a &drm_file. Use drm_client_add() to complete the process.
  * The caller needs to hold a reference on @dev before calling this function.
  * The client is freed when the &drm_device is unregistered. See drm_client_release().
  *
  * Returns:
  * Zero on success or negative error code on failure.
  */
-int drm_client_new(struct drm_device *dev, struct drm_client_dev *client,
-		   const char *name, const struct drm_client_funcs *funcs)
+int drm_client_init(struct drm_device *dev, struct drm_client_dev *client,
+		    const char *name, const struct drm_client_funcs *funcs)
 {
 	int ret;
 
@@ -95,10 +96,6 @@ int drm_client_new(struct drm_device *dev, struct drm_client_dev *client,
 	if (ret)
 		goto err_put_module;
 
-	mutex_lock(&dev->clientlist_mutex);
-	list_add(&client->list, &dev->clientlist);
-	mutex_unlock(&dev->clientlist_mutex);
-
 	drm_dev_get(dev);
 
 	return 0;
@@ -109,13 +106,33 @@ int drm_client_new(struct drm_device *dev, struct drm_client_dev *client,
 
 	return ret;
 }
-EXPORT_SYMBOL(drm_client_new);
+EXPORT_SYMBOL(drm_client_init);
+
+/**
+ * drm_client_add - Add client to the device list
+ * @client: DRM client
+ *
+ * Add the client to the &drm_device client list to activate its callbacks.
+ * @client must be initialized by a call to drm_client_init(). After
+ * drm_client_add() it is no longer permissible to call drm_client_release()
+ * directly (outside the unregister callback), instead cleanup will happen
+ * automatically on driver unload.
+ */
+void drm_client_add(struct drm_client_dev *client)
+{
+	struct drm_device *dev = client->dev;
+
+	mutex_lock(&dev->clientlist_mutex);
+	list_add(&client->list, &dev->clientlist);
+	mutex_unlock(&dev->clientlist_mutex);
+}
+EXPORT_SYMBOL(drm_client_add);
 
 /**
  * drm_client_release - Release DRM client resources
  * @client: DRM client
  *
- * Releases resources by closing the &drm_file that was opened by drm_client_new().
+ * Releases resources by closing the &drm_file that was opened by drm_client_init().
  * It is called automatically if the &drm_client_funcs.unregister callback is _not_ set.
  *
  * This function should only be called from the unregister callback. An exception

drivers/gpu/drm/drm_fb_cma_helper.c

@@ -160,7 +160,7 @@ struct drm_fbdev_cma *drm_fbdev_cma_init(struct drm_device *dev,
 
 	fb_helper = &fbdev_cma->fb_helper;
 
-	ret = drm_client_new(dev, &fb_helper->client, "fbdev", NULL);
+	ret = drm_client_init(dev, &fb_helper->client, "fbdev", NULL);
 	if (ret)
 		goto err_free;
 
@@ -169,6 +169,8 @@ struct drm_fbdev_cma *drm_fbdev_cma_init(struct drm_device *dev,
 	if (ret)
 		goto err_client_put;
 
+	drm_client_add(&fb_helper->client);
+
 	return fbdev_cma;
 
 err_client_put:

drivers/gpu/drm/drm_fb_helper.c

@@ -3218,12 +3218,14 @@ int drm_fbdev_generic_setup(struct drm_device *dev, unsigned int preferred_bpp)
 	if (!fb_helper)
 		return -ENOMEM;
 
-	ret = drm_client_new(dev, &fb_helper->client, "fbdev", &drm_fbdev_client_funcs);
+	ret = drm_client_init(dev, &fb_helper->client, "fbdev", &drm_fbdev_client_funcs);
 	if (ret) {
 		kfree(fb_helper);
 		return ret;
 	}
 
+	drm_client_add(&fb_helper->client);
+
 	fb_helper->preferred_bpp = preferred_bpp;
 
 	drm_fbdev_client_hotplug(&fb_helper->client);

include/drm/drm_client.h

@@ -87,9 +87,10 @@ struct drm_client_dev {
 	struct drm_file *file;
 };
 
-int drm_client_new(struct drm_device *dev, struct drm_client_dev *client,
-		   const char *name, const struct drm_client_funcs *funcs);
+int drm_client_init(struct drm_device *dev, struct drm_client_dev *client,
+		    const char *name, const struct drm_client_funcs *funcs);
 void drm_client_release(struct drm_client_dev *client);
+void drm_client_add(struct drm_client_dev *client);
 
 void drm_client_dev_unregister(struct drm_device *dev);
 void drm_client_dev_hotplug(struct drm_device *dev);