drm/bufs: Fix Spectre v1 vulnerability

idx can be 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_bufs.c:1420 drm_legacy_freebufs() warn: potential
spectre issue 'dma->buflist' [r] (local cap)

Fix this by sanitizing idx before using it to index dma->buflist

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

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/20181016095549.GA23586@embeddedor.com

drivers/gpu/drm/drm_bufs.c

@@ -36,6 +36,8 @@
 #include <drm/drmP.h>
 #include "drm_legacy.h"
 
+#include <linux/nospec.h>
+
 static struct drm_map_list *drm_find_matching_map(struct drm_device *dev,
 						  struct drm_local_map *map)
 {
@@ -1417,6 +1419,7 @@ int drm_legacy_freebufs(struct drm_device *dev, void *data,
 				  idx, dma->buf_count - 1);
 			return -EINVAL;
 		}
+		idx = array_index_nospec(idx, dma->buf_count);
 		buf = dma->buflist[idx];
 		if (buf->file_priv != file_priv) {
 			DRM_ERROR("Process %d freeing buffer not owned\n",