fbdev: Prevent possible use-after-free in fb_release()

Most fbdev drivers have issues with the fb_info lifetime, because call to
framebuffer_release() from their driver's .remove callback, rather than
doing from fbops.fb_destroy callback.

Doing that will destroy the fb_info too early, while references to it may
still exist, leading to a use-after-free error.

To prevent this, check the fb_info reference counter when attempting to
kfree the data structure in framebuffer_release(). That will leak it but
at least will prevent the mentioned error.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220505220413.365977-1-javierm@redhat.com

diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c
index 2689294..82e31a2 100644 (file)
--- a/drivers/video/fbdev/core/fbsysfs.c
+++ b/drivers/video/fbdev/core/fbsysfs.c
@@ -80,6 +80,10 @@ void framebuffer_release(struct fb_info *info)
 {
        if (!info)
                return;
+
+       if (WARN_ON(refcount_read(&info->count)))
+               return;
+
        kfree(info->apertures);
        kfree(info);
 }