drm/i915: Fix oops caused by fbdev initialization failure

intelfb_create() is called once on driver initialization. If it fails,
ifbdev->helper.fbdev, ifbdev->fb or ifbdev->fb->obj may be NULL.

Further up in the call stack, intel_fbdev_initial_config() calls
intel_fbdev_fini() to tear down the ifbdev on failure. This calls
intel_fbdev_destroy() which dereferences ifbdev->fb. Fix the ensuing
oops.

Also check in these functions if ifbdev is not NULL to avoid oops:

i915_gem_framebuffer_info() is called on access to debugfs file
"i915_gem_framebuffer" and dereferences ifbdev, ifbdev->helper.fb
and ifbdev->helper.fb->obj.

intel_connector_add_to_fbdev() / intel_connector_remove_from_fbdev()
are called when registering / unregistering an mst connector and
dereference ifbdev.

v3: Drop additional null pointer checks in intel_fbdev_set_suspend(),
    intel_fbdev_output_poll_changed() and intel_fbdev_restore_mode()
    since they already check if ifbdev is not NULL, which is sufficient
    now that intel_fbdev_fini() is called on initialization failure.
    (Requested by Daniel Vetter <daniel.vetter@ffwll.ch>)

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: http://patchwork.freedesktop.org/patch/msgid/d05f0edf121264a9d0adb8ca713fd8cc4ae068bf.1447938059.git.lukas@wunner.de
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

3 files changed, 25 insertions, 15 deletions

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index a9af884dadd5..08eb40292933 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -1878,17 +1878,19 @@ static int i915_gem_framebuffer_info(struct seq_file *m, void *data)
         struct drm_i915_private *dev_priv = dev->dev_private;
 
         ifbdev = dev_priv->fbdev;
-        fb = to_intel_framebuffer(ifbdev->helper.fb);
-
-        seq_printf(m, "fbcon size: %d x %d, depth %d, %d bpp, modifier 0x%llx, refcount %d, obj ",
-                   fb->base.width,
-                   fb->base.height,
-                   fb->base.depth,
-                   fb->base.bits_per_pixel,
-                   fb->base.modifier[0],
-                   atomic_read(&fb->base.refcount.refcount));
-        describe_obj(m, fb->obj);
-        seq_putc(m, '\n');
+        if (ifbdev) {
+                fb = to_intel_framebuffer(ifbdev->helper.fb);
+
+                seq_printf(m, "fbcon size: %d x %d, depth %d, %d bpp, modifier 0x%llx, refcount %d, obj ",
+                           fb->base.width,
+                           fb->base.height,
+                           fb->base.depth,
+                           fb->base.bits_per_pixel,
+                           fb->base.modifier[0],
+                           atomic_read(&fb->base.refcount.refcount));
+                describe_obj(m, fb->obj);
+                seq_putc(m, '\n');
+        }
 #endif
 
         mutex_lock(&dev->mode_config.fb_lock);
diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
index 8a604ac797aa..a12d1c7ee0e7 100644
--- a/drivers/gpu/drm/i915/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/intel_dp_mst.c
@@ -408,7 +408,10 @@ static void intel_connector_add_to_fbdev(struct intel_connector *connector)
 {
 #ifdef CONFIG_DRM_FBDEV_EMULATION
         struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
-        drm_fb_helper_add_one_connector(&dev_priv->fbdev->helper, &connector->base);
+
+        if (dev_priv->fbdev)
+                drm_fb_helper_add_one_connector(&dev_priv->fbdev->helper,
+                                                &connector->base);
 #endif
 }
 
@@ -416,7 +419,10 @@ static void intel_connector_remove_from_fbdev(struct intel_connector *connector)
 {
 #ifdef CONFIG_DRM_FBDEV_EMULATION
         struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
-        drm_fb_helper_remove_one_connector(&dev_priv->fbdev->helper, &connector->base);
+
+        if (dev_priv->fbdev)
+                drm_fb_helper_remove_one_connector(&dev_priv->fbdev->helper,
+                                                   &connector->base);
 #endif
 }
 
diff --git a/drivers/gpu/drm/i915/intel_fbdev.c b/drivers/gpu/drm/i915/intel_fbdev.c
index 98772d37b09f..cdbef32897f0 100644
--- a/drivers/gpu/drm/i915/intel_fbdev.c
+++ b/drivers/gpu/drm/i915/intel_fbdev.c
@@ -529,8 +529,10 @@ static void intel_fbdev_destroy(struct drm_device *dev,
 
         drm_fb_helper_fini(&ifbdev->helper);
 
-        drm_framebuffer_unregister_private(&ifbdev->fb->base);
-        drm_framebuffer_remove(&ifbdev->fb->base);
+        if (ifbdev->fb) {
+                drm_framebuffer_unregister_private(&ifbdev->fb->base);
+                drm_framebuffer_remove(&ifbdev->fb->base);
+        }
 }
 
 /*