[PATCH] drm/vmwgfx: Fix potential Spectre v1

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

arg.version 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/vmwgfx/vmwgfx_execbuf.c:4526 vmw_execbuf_ioctl() warn:
potential spectre issue 'copy_offset' [w]

Fix this by sanitizing arg.version before using it to index copy_offset

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>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c b/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
index 1f13457..ad91c6e 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
@@ -25,6 +25,7 @@
  *
  **************************************************************************/
 #include <linux/sync_file.h>
+#include <linux/nospec.h>
 
 #include "vmwgfx_drv.h"
 #include "vmwgfx_reg.h"
@@ -4520,8 +4521,10 @@ int vmw_execbuf_ioctl(struct drm_device *dev, unsigned long data,
 		return -EINVAL;
 	}
 
-	if (arg.version > 1 &&
-	    copy_from_user(&arg.context_handle,
+	if (arg.version >= ARRAY_SIZE(copy_offset))
+		return -EFAULT;
+	arg.version = array_index_nospec(arg.version, ARRAY_SIZE(copy_offset));
+	if (copy_from_user(&arg.context_handle,
 			   (void __user *) (data + copy_offset[0]),
 			   copy_offset[arg.version - 1] -
 			   copy_offset[0]) != 0)
-- 
2.7.4