On Wed, Aug 18, 2021 at 03:38:18PM +0800, Desmond Cheong Zhi Xi wrote:

There is a window after calling drm_master_release, and before a file is freed, where drm_file can have is_master set to true, but both the drm_file and drm_device have no master.

This could result in wrongly approving permissions in drm_is_current_master_locked. Add a check that fpriv->master is non-NULl to guard against this scenario.

Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com

This should be impossible, drm_master_release is only called when the struct file is released, which means all ioctls and anything else have finished (they hold a temporary reference).

fpriv->master can change (if the drm_file becomes newly minted master and wasnt one before through the setmaster ioctl), but it cannot become NULL before it's completely gone from the system. -Daniel

---

drivers/gpu/drm/drm_auth.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index 8c0e0dba1611..f9267b21556e 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -66,7 +66,8 @@ static bool drm_is_current_master_locked(struct drm_file *fpriv) lockdep_assert_once(lockdep_is_held(&fpriv->minor->dev->master_lookup_lock) || lockdep_is_held(&fpriv->minor->dev->master_mutex));

• return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;

• return (fpriv->is_master && fpriv->master &&

• drm_lease_owner(fpriv->master) == fpriv->minor->dev->master);
  

} /** -- 2.25.1