[PATCH 13/16] drm/fb-helper: Avoid race with DRM userspace

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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.

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

Suggested-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Signed-off-by: Noralf Trønnes <noralf@xxxxxxxxxxx>
---
 drivers/gpu/drm/drm_auth.c      | 20 ++++++++++++++
 drivers/gpu/drm/drm_fb_helper.c | 49 ++++++++++++++++++++++++---------
 drivers/gpu/drm/drm_internal.h  |  2 ++
 3 files changed, 58 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 1669c42c40ed..db199807b7dc 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -368,3 +368,23 @@ 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);
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 4a073cd4e423..9f253fcf3f79 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -41,6 +41,8 @@
 #include <drm/drm_crtc_helper.h>
 #include <drm/drm_atomic.h>
 
+#include "drm_internal.h"
+
 static bool drm_fbdev_emulation = true;
 module_param_named(fbdev_emulation, drm_fbdev_emulation, bool, 0600);
 MODULE_PARM_DESC(fbdev_emulation,
@@ -235,7 +237,12 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
 		return 0;
 
 	mutex_lock(&fb_helper->lock);
-	ret = drm_client_modesets_commit(fb_helper->dev, fb_helper->modesets);
+	if (drm_master_internal_acquire(fb_helper->dev)) {
+		ret = drm_client_modesets_commit(fb_helper->dev, fb_helper->modesets);
+		drm_master_internal_release(fb_helper->dev);
+	} else {
+		ret = -EBUSY;
+	}
 
 	do_delayed = fb_helper->delayed_hotplug;
 	if (do_delayed)
@@ -332,13 +339,16 @@ static struct sysrq_key_op sysrq_drm_fb_helper_restore_op = { };
 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))
-		drm_client_modesets_dpms(fb_helper->dev, fb_helper->modesets, dpms_mode);
+	if (drm_master_internal_acquire(dev)) {
+		drm_client_modesets_dpms(dev, fb_helper->modesets, dpms_mode);
+		drm_master_internal_release(dev);
+	}
 	mutex_unlock(&fb_helper->lock);
 }
 
@@ -1097,6 +1107,7 @@ static int setcmap_atomic(struct fb_cmap *cmap, struct fb_info *info)
 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)
@@ -1104,9 +1115,9 @@ 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)
@@ -1116,7 +1127,8 @@ 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;
@@ -1136,11 +1148,13 @@ 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_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;
 	}
@@ -1177,13 +1191,15 @@ 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;
 }
 EXPORT_SYMBOL(drm_fb_helper_ioctl);
@@ -1426,15 +1442,19 @@ 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;
@@ -1451,6 +1471,7 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
 	int ret = 0;
 	int crtc_count = 0;
 	struct drm_connector_list_iter conn_iter;
+	struct drm_device *dev = fb_helper->dev;
 	struct drm_fb_helper_surface_size sizes;
 	struct drm_connector *connector;
 	struct drm_mode_set *mode_set;
@@ -1593,8 +1614,10 @@ 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))
-			drm_client_modesets_commit(fb_helper->dev, fb_helper->modesets);
+		if (!fb_helper->deferred_setup && drm_master_internal_acquire(dev)) {
+			drm_client_modesets_commit(dev, fb_helper->modesets);
+			drm_master_internal_release(dev);
+		}
 		return -EAGAIN;
 	}
 
diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
index 251d67e04c2d..6d5d1184c084 100644
--- a/drivers/gpu/drm/drm_internal.h
+++ b/drivers/gpu/drm/drm_internal.h
@@ -91,6 +91,8 @@ 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;
-- 
2.20.1

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