drm/ioctl: Fix Spectre v1 vulnerabilities

nr is indirectly controlled by user-space, hence leading to a
potential exploitation of the Spectre variant 1 vulnerability.

This issue was detected with the help of Smatch:

drivers/gpu/drm/drm_ioctl.c:805 drm_ioctl() warn: potential spectre issue 'dev->driver->ioctls' [r]
drivers/gpu/drm/drm_ioctl.c:810 drm_ioctl() warn: potential spectre issue 'drm_ioctls' [r] (local cap)
drivers/gpu/drm/drm_ioctl.c:892 drm_ioctl_flags() warn: potential spectre issue 'drm_ioctls' [r] (local cap)

Fix this by sanitizing nr before using it to index dev->driver->ioctls
and drm_ioctls.

Notice that given that speculation windows are large, the policy is
to kill the speculation on the first load and not worry if it can be
completed with a dependent load/store [1].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181220000015.GA18973@embeddedor

drivers/gpu/drm/drm_ioctl.c

@@ -37,6 +37,7 @@
 
 #include <linux/pci.h>
 #include <linux/export.h>
+#include <linux/nospec.h>
 
 /**
  * DOC: getunique and setversion story
@@ -800,13 +801,17 @@ long drm_ioctl(struct file *filp,
 
 	if (is_driver_ioctl) {
 		/* driver ioctl */
-		if (nr - DRM_COMMAND_BASE >= dev->driver->num_ioctls)
+		unsigned int index = nr - DRM_COMMAND_BASE;
+
+		if (index >= dev->driver->num_ioctls)
 			goto err_i1;
-		ioctl = &dev->driver->ioctls[nr - DRM_COMMAND_BASE];
+		index = array_index_nospec(index, dev->driver->num_ioctls);
+		ioctl = &dev->driver->ioctls[index];
 	} else {
 		/* core ioctl */
 		if (nr >= DRM_CORE_IOCTL_COUNT)
 			goto err_i1;
+		nr = array_index_nospec(nr, DRM_CORE_IOCTL_COUNT);
 		ioctl = &drm_ioctls[nr];
 	}
 
@@ -888,6 +893,7 @@ bool drm_ioctl_flags(unsigned int nr, unsigned int *flags)
 
 	if (nr >= DRM_CORE_IOCTL_COUNT)
 		return false;
+	nr = array_index_nospec(nr, DRM_CORE_IOCTL_COUNT);
 
 	*flags = drm_ioctls[nr].flags;
 	return true;