drivers/misc/sgi-gru: fix Spectre v1 vulnerability

req.gid 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:

vers/misc/sgi-gru/grukdump.c:200 gru_dump_chiplet_request() warn:
potential spectre issue 'gru_base' [w]

Fix this by sanitizing req.gid before calling macro GID_TO_GRU, which
uses it to index gru_base.

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/sgi-gru/grukdump.c

@@ -27,6 +27,9 @@
 #include <linux/delay.h>
 #include <linux/bitops.h>
 #include <asm/uv/uv_hub.h>
+
+#include <linux/nospec.h>
+
 #include "gru.h"
 #include "grutables.h"
 #include "gruhandles.h"
@@ -196,6 +199,7 @@ int gru_dump_chiplet_request(unsigned long arg)
 	/* Currently, only dump by gid is implemented */
 	if (req.gid >= gru_max_gids)
 		return -EINVAL;
+	req.gid = array_index_nospec(req.gid, gru_max_gids);
 
 	gru = GID_TO_GRU(req.gid);
 	ubuf = req.buf;