[PATCH 3/3] drm: allow render capable master with DRM_AUTH ioctls

[Emil Velikov]

From: Emil Velikov <emil.velikov at collabora.com>

There are cases (in mesa and applications) where one would open the
primary node without properly authenticating the client.

Sometimes we don't check if the authentication succeeds, but there's
also cases we simply forget to do it.

The former was a case for Mesa where it did not not check the return
value of drmGetMagic() [1]. That was fixed recently although, there's
the question of older drivers or other apps that exbibit this behaviour.

While omitting the call results in issues as seen in [2] and [3].

In the libva case, libva itself doesn't authenticate the DRM client and
the vaGetDisplayDRM documentation doesn't mention if the app should
either.

As of today, the official vainfo utility doesn't authenticate.

To workaround issues like these, some users resort to running their apps
under sudo. Which admittedly isn't always a good idea.

Since any DRIVER_RENDER driver has sufficient isolation between clients,
we can use that, for unauthenticated [primary node] ioctls that require
DRM_AUTH. But only if the respective ioctl is tagged as DRM_RENDER_ALLOW.

v2:
- Rework/simplify if check (Daniel V)
- Add examples to commit messages, elaborate. (Daniel V)

v3:
- Use single unlikely (Daniel V)

v4:
- Reapply patch, use DRIVER_FORCE_AUTH w/a for amdgpu/radeon.

[1] https://gitlab.freedesktop.org/mesa/mesa/blob/2bc1f5c2e70fe3b4d41f060af9859bc2a94c5b62/src/egl/drivers/dri2/platform_wayland.c#L1136
[2] https://lists.freedesktop.org/archives/libva/2016-July/004185.html
[3] https://gitlab.freedesktop.org/mesa/kmscube/issues/1
Testcase: igt/core_unauth_vs_render
Cc: intel-gfx at lists.freedesktop.org
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov at collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/drm_ioctl.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 09f7f8e33fa3..c30feb5e97e3 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -517,6 +517,13 @@ int drm_version(struct drm_device *dev, void *data,
 	return err;
 }
 
+static inline bool
+drm_render_driver_and_ioctl(const struct drm_device *dev, u32 flags)
+{
+	return drm_core_check_feature(dev, DRIVER_RENDER) &&
+		(flags & DRM_RENDER_ALLOW);
+}
+
 /**
  * drm_ioctl_permit - Check ioctl permissions against caller
  *
@@ -531,6 +538,8 @@ int drm_version(struct drm_device *dev, void *data,
  */
 int drm_ioctl_permit(u32 flags, struct drm_file *file_priv)
 {
+	const struct drm_device *dev = file_priv->minor->dev;
+
 	/* ROOT_ONLY is only for CAP_SYS_ADMIN */
 	if (unlikely((flags & DRM_ROOT_ONLY) && !capable(CAP_SYS_ADMIN)))
 		return -EACCES;
@@ -538,7 +547,14 @@ int drm_ioctl_permit(u32 flags, struct drm_file *file_priv)
 	/* AUTH is only for authenticated or render client */
 	if (unlikely((flags & DRM_AUTH) && !drm_is_render_client(file_priv) &&
 		     !file_priv->authenticated))
-		return -EACCES;
+		/*
+		 * Although we allow:
+		 *  - render drivers with DRM_RENDER_ALLOW ioctls, when
+		 *  - drivers do not explicitly mandate authentication.
+		 */
+		if (!drm_render_driver_and_ioctl(dev, flags) ||
+		    drm_core_check_feature(dev, DRIVER_FORCE_AUTH))
+			return -EACCES;
 
 	/* MASTER is only for master or control clients */
 	if (unlikely((flags & DRM_MASTER) &&
-- 
2.21.0