[PATCH 1/2] drm: Add a locked version of drm_is_current_master

[Emil Velikov]

On Sat, 12 Jun 2021 at 13:55, Desmond Cheong Zhi Xi
<desmondcheongzx at gmail.com> wrote:
>
> While checking the master status of the DRM file in
> drm_is_current_master(), the device's master mutex should be
> held. Without the mutex, the pointer fpriv->master may be freed
> concurrently by another process calling drm_setmaster_ioctl(). This
> could lead to use-after-free errors when the pointer is subsequently
> dereferenced in drm_lease_owner().
>
> The callers of drm_is_current_master() from drm_auth.c hold the
> device's master mutex, but external callers do not. Hence, we implement
> drm_is_current_master_locked() to be used within drm_auth.c, and
> modify drm_is_current_master() to grab the device's master mutex
> before checking the master status.
>
> Reported-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx at gmail.com>

Reviewed-by: Emil Velikov <emil.l.velikov at gmail.com>

-Emil