[PATCH] drm/vkms: fix use-after-free when drm_gem_handle_create() fails

[Chris Wilson]

Quoting Eric Biggers (2019-02-26 22:08:58)
> From: Eric Biggers <ebiggers at google.com>
> 
> If drm_gem_handle_create() fails in vkms_gem_create(), then the
> vkms_gem_object is freed twice: once when the reference is dropped by
> drm_gem_object_put_unlocked(), and again by the extra calls to
> drm_gem_object_release() and kfree().
> 
> Fix it by skipping the second release and free.
> 
> This bug was originally found in the vgem driver by syzkaller using
> fault injection, but I noticed it's also present in the vkms driver.
> 
> Fixes: 559e50fd34d1 ("drm/vkms: Add dumb operations")
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
> Cc: Haneen Mohammed <hamohammed.sa at gmail.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: stable at vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers at google.com>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris