drm/fb_helper: Fix references to dev->mode_config.num_connector

commit 255f0e7c418ad95a4baeda017ae6182ba9b3c423 upstream.

During boot, MST hotplugs are generally expected (even if no physical
hotplugging occurs) and result in DRM's connector topology changing.
This means that using num_connector from the current mode configuration
can lead to the number of connectors changing under us. This can lead to
some nasty scenarios in fbcon:

- We allocate an array to the size of dev->mode_config.num_connectors.
- MST hotplug occurs, dev->mode_config.num_connectors gets incremented.
- We try to loop through each element in the array using the new value
  of dev->mode_config.num_connectors, and end up going out of bounds
  since dev->mode_config.num_connectors is now larger then the array we
  allocated.

fb_helper->connector_count however, will always remain consistent while
we do a modeset in fb_helper.

Note: This is just polish for 4.7, Dave Airlie's drm_connector
refcounting fixed these bugs for real. But it's good enough duct-tape
for stable kernel backporting, since backporting the refcounting
changes is way too invasive.

Signed-off-by: Lyude <cpaul@redhat.com>
[danvet: Clarify why we need this. Also remove the now unused "dev"
local variable to appease gcc.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1463065021-18280-3-git-send-email-cpaul@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 69cbab5e5c81f29008247776d81447f21aede835..5ad036741b9945e4deb0336a9476f043a4fff94a 100644 (file)
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1899,7 +1899,6 @@ static int drm_pick_crtcs(struct drm_fb_helper *fb_helper,
                          int n, int width, int height)
 {
        int c, o;
-       struct drm_device *dev = fb_helper->dev;
        struct drm_connector *connector;
        const struct drm_connector_helper_funcs *connector_funcs;
        struct drm_encoder *encoder;
@@ -1918,7 +1917,7 @@ static int drm_pick_crtcs(struct drm_fb_helper *fb_helper,
        if (modes[n] == NULL)
                return best_score;
 
-       crtcs = kzalloc(dev->mode_config.num_connector *
+       crtcs = kzalloc(fb_helper->connector_count *
                        sizeof(struct drm_fb_helper_crtc *), GFP_KERNEL);
        if (!crtcs)
                return best_score;
@@ -1964,7 +1963,7 @@ static int drm_pick_crtcs(struct drm_fb_helper *fb_helper,
                if (score > best_score) {
                        best_score = score;
                        memcpy(best_crtcs, crtcs,
-                              dev->mode_config.num_connector *
+                              fb_helper->connector_count *
                               sizeof(struct drm_fb_helper_crtc *));
                }
        }