[PATCH] drm: Fix an unwanted master inheritance

[Emil Velikov]

Hi Thomas,

Something doesn't feel quite right, please read on.

On 30 November 2015 at 12:44, Thomas Hellstrom <thellstrom at vmware.com> wrote:
> A client calling drmSetMaster() using a file descriptor that was opened
> when another client was master would inherit the latter client's master
> object and all it's authenticated clients.
>
> This is unwanted behaviour, and when this happens, instead allocate a
> brand new master object for the client calling drmSetMaster().
>
> Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>
> ---
>  drivers/gpu/drm/drm_drv.c  | 12 +++++++
>  drivers/gpu/drm/drm_fops.c | 80 ++++++++++++++++++++++++++++++----------------
>  include/drm/drmP.h         |  6 ++++
>  3 files changed, 70 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 9362609..1f072ba 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -160,6 +160,18 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
>                 goto out_unlock;
>         }
>
> +       if (!file_priv->allowed_master) {
> +               struct drm_master *saved_master = file_priv->master;
> +
> +               ret = drm_new_set_master(dev, file_priv);
> +               if (ret)
> +                       file_priv->master = saved_master;
Imho this shouldn'e belong here but in drm_new_set_master() - i.e. it
should unwind things on error. Similarly, although we restore the old
drm_master (below), we still have is_master, allowed_master and
authenticated set. Thus one can reuse the elevated credentials (is
this the right terminology?) despite that the ioctl has failed.

> +               else
> +                       drm_master_put(&saved_master);
> +
> +               goto out_unlock;
> +       }
> +
>         file_priv->minor->master = drm_master_get(file_priv->master);
>         file_priv->is_master = 1;
>         if (dev->driver->master_set) {
> diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
> index c59ce4d..4b5c11c 100644
> --- a/drivers/gpu/drm/drm_fops.c
> +++ b/drivers/gpu/drm/drm_fops.c
> @@ -126,6 +126,56 @@ static int drm_cpu_valid(void)
>  }
>
>  /**
> + * drm_new_set_master - Allocate a new master object and become master for the
> + * associated master realm.
> + *
> + * @dev: The associated device.
> + * @fpriv: File private identifying the client.
> + *
> + * This function must be called with dev::struct_mutex held. Returns negative
> + * error code on failure, zero on success.
> + */
> +int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
> +{
> +       int ret;
> +
> +       lockdep_assert_held_once(&dev->master_mutex);
> +       /* create a new master */
> +       fpriv->minor->master = drm_master_create(fpriv->minor);
> +       if (!fpriv->minor->master)
> +               return -ENOMEM;
> +
> +       fpriv->is_master = 1;
> +       fpriv->allowed_master = 1;
> +
> +       /* take another reference for the copy in the local file priv */
> +       fpriv->master = drm_master_get(fpriv->minor->master);
> +
> +       fpriv->authenticated = 1;
> +
> +       if (dev->driver->master_create) {
> +               ret = dev->driver->master_create(dev, fpriv->master);
> +               if (ret) {
> +                       /* drop both references if this fails */
> +                       drm_master_put(&fpriv->minor->master);
> +                       drm_master_put(&fpriv->master);
> +                       return ret;
> +               }
> +       }
> +       if (dev->driver->master_set) {
> +               ret = dev->driver->master_set(dev, fpriv, true);
> +               if (ret) {
Afaics both of these callbacks are available from legacy(UMS) drivers
aren't they ? With the radeon UMS removal patches in the works, this
leaves vmwgfx.

Either way, perhaps we should set is_master, allowed_master and
authenticated after these two ? Or alternatively restore the original
values on error.

Did I miss something or the above sounds about right ?

Regards,
Emil