[RFT PATCH] drm/fb-helper: Don't deallocate fb colormap and free fb info too early

[Javier Martinez Canillas]

Currently these are done in drm_fb_helper_fini() but this helper is called
by drivers in their .remove callback, which could lead to a use-after-free
if a process has opened the emulated fbdev node while a driver is removed.

For example, in i915 driver the call chain during remove is the following:

struct pci_driver i915_pci_driver = {
...
        .remove = i915_pci_remove,
...
};

i915_pci_remove
  i915_driver_remove
    intel_modeset_driver_remove_noirq
      intel_fbdev_fini
        intel_fbdev_destroy
          drm_fb_helper_fini
            framebuffer_release

Later the process will close the fbdev node file descriptor leading to the
mentioned use-after-free bug in drm_fbdev_fb_destroy(), due the following:

drm_fbdev_fb_destroy
  drm_fbdev_release(info->par); <-- info was already freed on .remove

To prevent that, let's move the framebuffer_release() call to the end of
the drm_fbdev_fb_destroy() function.

Also, the call to fb_dealloc_cmap() in drm_fb_helper_fini() is too early
and is more correct to do it in drm_fbdev_fb_destroy() as well. After a
call to drm_fbdev_release() has been made.

Reported-by: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
---
 drivers/gpu/drm/drm_fb_helper.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index d265a73313c9..7288fbd26bcc 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -627,12 +627,6 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
 	cancel_work_sync(&fb_helper->resume_work);
 	cancel_work_sync(&fb_helper->damage_work);
 
-	info = fb_helper->fbdev;
-	if (info) {
-		if (info->cmap.len)
-			fb_dealloc_cmap(&info->cmap);
-		framebuffer_release(info);
-	}
 	fb_helper->fbdev = NULL;
 
 	mutex_lock(&kernel_fb_helper_lock);
@@ -2112,6 +2106,9 @@ static void drm_fbdev_release(struct drm_fb_helper *fb_helper)
 static void drm_fbdev_fb_destroy(struct fb_info *info)
 {
 	drm_fbdev_release(info->par);
+	if (info->cmap.len)
+		fb_dealloc_cmap(&info->cmap);
+	framebuffer_release(info);
 }
 
 static int drm_fbdev_fb_mmap(struct fb_info *info, struct vm_area_struct *vma)
-- 
2.35.1