Re: [PATCH v2 1/2] drm: Add a locked version of drm_is_current_master

Daniel Vetter
2021-06-17T17:03:43Z

On Tue, Jun 15, 2021 at 10:36:44AM +0800, Desmond Cheong Zhi Xi 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@ffwll.ch>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
> ---
> drivers/gpu/drm/drm_auth.c | 23 +++++++++++++++++++----
> 1 file changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index 232abbba3686..c6bf52c310a9 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -61,6 +61,8 @@
> * trusted clients.
> */
>
> +static bool drm_is_current_master_locked(struct drm_file *fpriv);

A bit a bikeshed, but we try to avoid forward declarations when they're
not needed. If you don't want to tear apart drm_is_current_master and the
_locked version then just move them together.

Can you pls do that and respin?

Otherwise looks all great.
-Daniel

> +
> int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
> {
> struct drm_auth *auth = data;
> @@ -223,7 +225,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
> if (ret)
> goto out_unlock;
>
> - if (drm_is_current_master(file_priv))
> + if (drm_is_current_master_locked(file_priv))
> goto out_unlock;
>
> if (dev->master) {
> @@ -272,7 +274,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
> if (ret)
> goto out_unlock;
>
> - if (!drm_is_current_master(file_priv)) {
> + if (!drm_is_current_master_locked(file_priv)) {
> ret = -EINVAL;
> goto out_unlock;
> }
> @@ -321,7 +323,7 @@ void drm_master_release(struct drm_file *file_priv)
> if (file_priv->magic)
> idr_remove(&file_priv->master->magic_map, file_priv->magic);
>
> - if (!drm_is_current_master(file_priv))
> + if (!drm_is_current_master_locked(file_priv))
> goto out;
>
> drm_legacy_lock_master_cleanup(dev, master);
> @@ -342,6 +344,13 @@ void drm_master_release(struct drm_file *file_priv)
> mutex_unlock(&dev->master_mutex);
> }
>
> +static bool drm_is_current_master_locked(struct drm_file *fpriv)
> +{
> + lockdep_assert_held_once(&fpriv->master->dev->master_mutex);
> +
> + return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> +}
> +
> /**
> * drm_is_current_master - checks whether @priv is the current master
> * @fpriv: DRM file private
> @@ -354,7 +363,13 @@ void drm_master_release(struct drm_file *file_priv)
> */
> bool drm_is_current_master(struct drm_file *fpriv)
> {
> - return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> + bool ret;
> +
> + mutex_lock(&fpriv->master->dev->master_mutex);
> + ret = drm_is_current_master_locked(fpriv);
> + mutex_unlock(&fpriv->master->dev->master_mutex);
> +
> + return ret;
> }
> EXPORT_SYMBOL(drm_is_current_master);
>
> --
> 2.25.1
>

--
Daniel Vetter
Software Engineer, Intel Corporation