drm: Return -ENOTTY for non-drm ioctls

drm: Return -ENOTTY for non-drm ioctls

Return -ENOTTY from drm_ioctl() when userspace passes in a cmd number
which doesn't relate to the drm subsystem.

Glibc uses the TCGETS ioctl to implement isatty(), and without this
change isatty() returns it incorrectly returns true for drm devices.

To test run this command:
$ if [ -t 0 ]; then echo is a tty; fi < /dev/dri/card0
which shows "is a tty" without this patch.

This may also modify memory which the userspace application is not
expecting.

Signed-off-by: Charles Baylis <cb-kernel@fishzet.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/YPG3IBlzaMhfPqCr@stando.fishzet.co.uk

drivers/gpu/drm/drm_ioctl.c

@@ -834,6 +834,9 @@ long drm_ioctl(struct file *filp,
 	if (drm_dev_is_unplugged(dev))
 		return -ENODEV;
 
+       if (DRM_IOCTL_TYPE(cmd) != DRM_IOCTL_BASE)
+               return -ENOTTY;
+
 	is_driver_ioctl = nr >= DRM_COMMAND_BASE && nr < DRM_COMMAND_END;
 
 	if (is_driver_ioctl) {

include/drm/drm_ioctl.h

@@ -68,6 +68,7 @@ typedef int drm_ioctl_compat_t(struct file *filp, unsigned int cmd,
 			       unsigned long arg);
 
 #define DRM_IOCTL_NR(n)                _IOC_NR(n)
+#define DRM_IOCTL_TYPE(n)              _IOC_TYPE(n)
 #define DRM_MAJOR       226
 
 /**