[PATCH AUTOSEL 4.9 001/107] drm/bufs: Fix Spectre v1 vulnerability

[Sasha Levin <sashal@kernel.org>]

From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>

[ Upstream commit a37805098900a6e73a55b3a43b7d3bcd987bb3f4 ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/gpu/drm/drm_bufs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/drm_bufs.c b/drivers/gpu/drm/drm_bufs.c
index adb1dd7fde5f..9ccd7d702cd3 100644
--- a/drivers/gpu/drm/drm_bufs.c
+++ b/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)
 {
@@ -1413,6 +1415,7 @@ 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",
-- 
2.19.1

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