drm/fb-helper: Avoid race with DRM userspace

drm_fb_helper_is_bound() is used to check if DRM userspace is in control.
This is done by looking at the fb on the primary plane. By the time
fb-helper gets around to committing, it's possible that the facts have
changed.

Avoid this race by holding the drm_device->master_mutex lock while
committing. When DRM userspace does its first open, it will now wait
until fb-helper is done. The helper will stay away if there's a master.

Two igt tests fail with the new 'bail out if master' rule. Work around
this by relaxing this rule for drm_fb_helper_restore_fbdev_mode_unlocked()
until the tests have been fixed. Add todo entry for this.

Locking rule: Always take the fb-helper lock first.

v5: drm_fb_helper_restore_fbdev_mode_unlocked(): Use
    restore_fbdev_mode_force()

v2:
- Remove drm_fb_helper_is_bound() (Daniel Vetter)
- No need to check fb_helper->dev->master in
  drm_fb_helper_single_fb_probe(), restore_fbdev_mode() has the check.

Suggested-by: 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/20190506180139.6913-3-noralf@tronnes.org

Documentation/gpu/todo.rst

@@ -281,6 +281,14 @@ it to use drm_mode_hsync() instead.
 
 Contact: Sean Paul
 
+drm_fb_helper tasks
+-------------------
+
+- drm_fb_helper_restore_fbdev_mode_unlocked() should call restore_fbdev_mode()
+  not the _force variant so it can bail out if there is a master. But first
+  these igt tests need to be fixed: kms_fbcon_fbt@psr and
+  kms_fbcon_fbt@psr-suspend.
+
 Core refactorings
 =================
 

drivers/gpu/drm/drm_auth.c

@@ -351,3 +351,23 @@ void drm_master_put(struct drm_master **master)
 	*master = NULL;
 }
 EXPORT_SYMBOL(drm_master_put);
+
+/* Used by drm_client and drm_fb_helper */
+bool drm_master_internal_acquire(struct drm_device *dev)
+{
+	mutex_lock(&dev->master_mutex);
+	if (dev->master) {
+		mutex_unlock(&dev->master_mutex);
+		return false;
+	}
+
+	return true;
+}
+EXPORT_SYMBOL(drm_master_internal_acquire);
+
+/* Used by drm_client and drm_fb_helper */
+void drm_master_internal_release(struct drm_device *dev)
+{
+	mutex_unlock(&dev->master_mutex);
+}
+EXPORT_SYMBOL(drm_master_internal_release);

drivers/gpu/drm/drm_fb_helper.c

@@ -44,6 +44,7 @@
 
 #include "drm_crtc_internal.h"
 #include "drm_crtc_helper_internal.h"
+#include "drm_internal.h"
 
 static bool drm_fbdev_emulation = true;
 module_param_named(fbdev_emulation, drm_fbdev_emulation, bool, 0600);
@@ -509,7 +510,7 @@ out:
 	return ret;
 }
 
-static int restore_fbdev_mode(struct drm_fb_helper *fb_helper)
+static int restore_fbdev_mode_force(struct drm_fb_helper *fb_helper)
 {
 	struct drm_device *dev = fb_helper->dev;
 
@@ -519,6 +520,21 @@ static int restore_fbdev_mode(struct drm_fb_helper *fb_helper)
 		return restore_fbdev_mode_legacy(fb_helper);
 }
 
+static int restore_fbdev_mode(struct drm_fb_helper *fb_helper)
+{
+	struct drm_device *dev = fb_helper->dev;
+	int ret;
+
+	if (!drm_master_internal_acquire(dev))
+		return -EBUSY;
+
+	ret = restore_fbdev_mode_force(fb_helper);
+
+	drm_master_internal_release(dev);
+
+	return ret;
+}
+
 /**
  * drm_fb_helper_restore_fbdev_mode_unlocked - restore fbdev configuration
  * @fb_helper: driver-allocated fbdev helper, can be NULL
@@ -542,7 +558,17 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
 		return 0;
 
 	mutex_lock(&fb_helper->lock);
-	ret = restore_fbdev_mode(fb_helper);
+	/*
+	 * TODO:
+	 * We should bail out here if there is a master by dropping _force.
+	 * Currently these igt tests fail if we do that:
+	 * - kms_fbcon_fbt@psr
+	 * - kms_fbcon_fbt@psr-suspend
+	 *
+	 * So first these tests need to be fixed so they drop master or don't
+	 * have an fd open.
+	 */
+	ret = restore_fbdev_mode_force(fb_helper);
 
 	do_delayed = fb_helper->delayed_hotplug;
 	if (do_delayed)
@@ -556,34 +582,6 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
 }
 EXPORT_SYMBOL(drm_fb_helper_restore_fbdev_mode_unlocked);
 
-static bool drm_fb_helper_is_bound(struct drm_fb_helper *fb_helper)
-{
-	struct drm_device *dev = fb_helper->dev;
-	struct drm_crtc *crtc;
-	int bound = 0, crtcs_bound = 0;
-
-	/*
-	 * Sometimes user space wants everything disabled, so don't steal the
-	 * display if there's a master.
-	 */
-	if (READ_ONCE(dev->master))
-		return false;
-
-	drm_for_each_crtc(crtc, dev) {
-		drm_modeset_lock(&crtc->mutex, NULL);
-		if (crtc->primary->fb)
-			crtcs_bound++;
-		if (crtc->primary->fb == fb_helper->fb)
-			bound++;
-		drm_modeset_unlock(&crtc->mutex);
-	}
-
-	if (bound < crtcs_bound)
-		return false;
-
-	return true;
-}
-
 #ifdef CONFIG_MAGIC_SYSRQ
 /*
  * restore fbcon display for all kms driver's using this helper, used for sysrq
@@ -604,7 +602,7 @@ static bool drm_fb_helper_force_kernel_mode(void)
 			continue;
 
 		mutex_lock(&helper->lock);
-		ret = restore_fbdev_mode(helper);
+		ret = restore_fbdev_mode_force(helper);
 		if (ret)
 			error = true;
 		mutex_unlock(&helper->lock);
@@ -663,20 +661,22 @@ static void dpms_legacy(struct drm_fb_helper *fb_helper, int dpms_mode)
 static void drm_fb_helper_dpms(struct fb_info *info, int dpms_mode)
 {
 	struct drm_fb_helper *fb_helper = info->par;
+	struct drm_device *dev = fb_helper->dev;
 
 	/*
 	 * For each CRTC in this fb, turn the connectors on/off.
 	 */
 	mutex_lock(&fb_helper->lock);
-	if (!drm_fb_helper_is_bound(fb_helper)) {
-		mutex_unlock(&fb_helper->lock);
-		return;
-	}
+	if (!drm_master_internal_acquire(dev))
+		goto unlock;
 
-	if (drm_drv_uses_atomic_modeset(fb_helper->dev))
+	if (drm_drv_uses_atomic_modeset(dev))
 		restore_fbdev_mode_atomic(fb_helper, dpms_mode == DRM_MODE_DPMS_ON);
 	else
 		dpms_legacy(fb_helper, dpms_mode);
+
+	drm_master_internal_release(dev);
+unlock:
 	mutex_unlock(&fb_helper->lock);
 }
 
@@ -1509,6 +1509,7 @@ backoff:
 int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info)
 {
 	struct drm_fb_helper *fb_helper = info->par;
+	struct drm_device *dev = fb_helper->dev;
 	int ret;
 
 	if (oops_in_progress)
@@ -1516,9 +1517,9 @@ int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info)
 
 	mutex_lock(&fb_helper->lock);
 
-	if (!drm_fb_helper_is_bound(fb_helper)) {
+	if (!drm_master_internal_acquire(dev)) {
 		ret = -EBUSY;
-		goto out;
+		goto unlock;
 	}
 
 	if (info->fix.visual == FB_VISUAL_TRUECOLOR)
@@ -1528,7 +1529,8 @@ int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info)
 	else
 		ret = setcmap_legacy(cmap, info);
 
-out:
+	drm_master_internal_release(dev);
+unlock:
 	mutex_unlock(&fb_helper->lock);
 
 	return ret;
@@ -1548,12 +1550,13 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
 			unsigned long arg)
 {
 	struct drm_fb_helper *fb_helper = info->par;
+	struct drm_device *dev = fb_helper->dev;
 	struct drm_mode_set *mode_set;
 	struct drm_crtc *crtc;
 	int ret = 0;
 
 	mutex_lock(&fb_helper->lock);
-	if (!drm_fb_helper_is_bound(fb_helper)) {
+	if (!drm_master_internal_acquire(dev)) {
 		ret = -EBUSY;
 		goto unlock;
 	}
@@ -1591,11 +1594,12 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
 		}
 
 		ret = 0;
-		goto unlock;
+		break;
 	default:
 		ret = -ENOTTY;
 	}
 
+	drm_master_internal_release(dev);
 unlock:
 	mutex_unlock(&fb_helper->lock);
 	return ret;
@@ -1847,15 +1851,18 @@ int drm_fb_helper_pan_display(struct fb_var_screeninfo *var,
 		return -EBUSY;
 
 	mutex_lock(&fb_helper->lock);
-	if (!drm_fb_helper_is_bound(fb_helper)) {
-		mutex_unlock(&fb_helper->lock);
-		return -EBUSY;
+	if (!drm_master_internal_acquire(dev)) {
+		ret = -EBUSY;
+		goto unlock;
 	}
 
 	if (drm_drv_uses_atomic_modeset(dev))
 		ret = pan_display_atomic(var, info);
 	else
 		ret = pan_display_legacy(var, info);
+
+	drm_master_internal_release(dev);
+unlock:
 	mutex_unlock(&fb_helper->lock);
 
 	return ret;
@@ -2014,7 +2021,7 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
 		DRM_INFO("Cannot find any crtc or sizes\n");
 
 		/* First time: disable all crtc's.. */
-		if (!fb_helper->deferred_setup && !READ_ONCE(fb_helper->dev->master))
+		if (!fb_helper->deferred_setup)
 			restore_fbdev_mode(fb_helper);
 		return -EAGAIN;
 	}
@@ -3029,6 +3036,7 @@ EXPORT_SYMBOL(drm_fb_helper_initial_config);
  */
 int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
 {
+	struct drm_device *dev = fb_helper->dev;
 	int err = 0;
 
 	if (!drm_fbdev_emulation || !fb_helper)
@@ -3041,12 +3049,14 @@ int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
 		return err;
 	}
 
-	if (!fb_helper->fb || !drm_fb_helper_is_bound(fb_helper)) {
+	if (!fb_helper->fb || !drm_master_internal_acquire(dev)) {
 		fb_helper->delayed_hotplug = true;
 		mutex_unlock(&fb_helper->lock);
 		return err;
 	}
 
+	drm_master_internal_release(dev);
+
 	DRM_DEBUG_KMS("\n");
 
 	drm_setup_crtcs(fb_helper, fb_helper->fb->width, fb_helper->fb->height);

drivers/gpu/drm/drm_internal.h

@@ -93,6 +93,8 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
 			 struct drm_file *file_priv);
 int drm_master_open(struct drm_file *file_priv);
 void drm_master_release(struct drm_file *file_priv);
+bool drm_master_internal_acquire(struct drm_device *dev);
+void drm_master_internal_release(struct drm_device *dev);
 
 /* drm_sysfs.c */
 extern struct class *drm_class;