[PATCH V2] IB/core: Add mitigation for Spectre V1

["Luck, Tony" <tony.luck@intel.com>]

Some processors may mispredict an array bounds check and
speculatively access memory that they should not. With
a user supplied array index we like to play things safe
by masking the value with the array size before it is
used as an index.

Signed-off-by: Tony Luck <tony.luck@intel.com>

---
V2: Mask the index *AFTER* the bounds check. Issue pointed
    out by Gustavo. Fix suggested by Ira.

 drivers/infiniband/core/user_mad.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/infiniband/core/user_mad.c b/drivers/infiniband/core/user_mad.c
index 9f8a48016b41..32cea5ed9ce1 100644
--- a/drivers/infiniband/core/user_mad.c
+++ b/drivers/infiniband/core/user_mad.c
@@ -49,6 +49,7 @@
 #include <linux/sched.h>
 #include <linux/semaphore.h>
 #include <linux/slab.h>
+#include <linux/nospec.h>
 
 #include <linux/uaccess.h>
 
@@ -888,7 +889,12 @@ static int ib_umad_unreg_agent(struct ib_umad_file *file, u32 __user *arg)
 	mutex_lock(&file->port->file_mutex);
 	mutex_lock(&file->mutex);
 
-	if (id >= IB_UMAD_MAX_AGENTS || !__get_agent(file, id)) {
+	if (id >= IB_UMAD_MAX_AGENTS) {
+		ret = -EINVAL;
+		goto out;
+	}
+	id = array_index_nospec(id, IB_UMAD_MAX_AGENTS);
+	if (!__get_agent(file, id)) {
 		ret = -EINVAL;
 		goto out;
 	}
-- 
2.20.1